Remove unused setTransactionType method (#31008)

diff --git a/proxy/backend/core/src/main/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatus.java b/proxy/backend/core/src/main/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatus.java
index 014a9d3..290c28c 100644
--- a/proxy/backend/core/src/main/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatus.java
+++ b/proxy/backend/core/src/main/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatus.java
@@ -19,9 +19,7 @@
 
 import lombok.Getter;
 import lombok.Setter;
-import org.apache.shardingsphere.infra.exception.core.ShardingSpherePreconditions;
 import org.apache.shardingsphere.transaction.api.TransactionType;
-import org.apache.shardingsphere.transaction.exception.SwitchTypeInTransactionException;
 
 /**
  * Transaction status.
@@ -32,7 +30,7 @@
     @Setter
     private volatile boolean inTransaction;
     
-    private volatile TransactionType transactionType;
+    private final TransactionType transactionType;
     
     @Setter
     private volatile boolean exceptionOccur;
@@ -42,16 +40,6 @@
     }
     
     /**
-     * Change transaction type of current channel.
-     *
-     * @param transactionType transaction type
-     */
-    public void setTransactionType(final TransactionType transactionType) {
-        ShardingSpherePreconditions.checkState(!inTransaction, SwitchTypeInTransactionException::new);
-        this.transactionType = transactionType;
-    }
-    
-    /**
      * Judge whether in connection held transaction.
      * 
      * @return is in connection held transaction or not
diff --git a/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/ConnectionSessionTest.java b/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/ConnectionSessionTest.java
index 845ae3c..bf1e56d 100644
--- a/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/ConnectionSessionTest.java
+++ b/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/ConnectionSessionTest.java
@@ -27,7 +27,6 @@
 import org.apache.shardingsphere.test.mock.AutoMockExtension;
 import org.apache.shardingsphere.test.mock.StaticMockSettings;
 import org.apache.shardingsphere.transaction.api.TransactionType;
-import org.apache.shardingsphere.transaction.exception.SwitchTypeInTransactionException;
 import org.apache.shardingsphere.transaction.rule.TransactionRule;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -43,7 +42,6 @@
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
-import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.mockito.Mockito.RETURNS_DEEP_STUBS;
 import static org.mockito.Mockito.mock;
@@ -72,15 +70,6 @@
     }
     
     @Test
-    void assertFailedSwitchTransactionTypeWhileBegin() {
-        connectionSession.setCurrentDatabase("db");
-        ContextManager contextManager = mockContextManager();
-        when(ProxyContext.getInstance().getContextManager()).thenReturn(contextManager);
-        new BackendTransactionManager(databaseConnectionManager).begin();
-        assertThrows(SwitchTypeInTransactionException.class, () -> connectionSession.getTransactionStatus().setTransactionType(TransactionType.XA));
-    }
-    
-    @Test
     void assertSwitchSchemaWhileBegin() {
         connectionSession.setCurrentDatabase("db");
         ContextManager contextManager = mockContextManager();
diff --git a/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatusTest.java b/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatusTest.java
index 370c693..bfff4b7 100644
--- a/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatusTest.java
+++ b/proxy/backend/core/src/test/java/org/apache/shardingsphere/proxy/backend/session/transaction/TransactionStatusTest.java
@@ -21,8 +21,6 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
-import static org.hamcrest.CoreMatchers.is;
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 class TransactionStatusTest {
@@ -35,13 +33,6 @@
     }
     
     @Test
-    void assertSetTransactionType() {
-        transactionStatus.setTransactionType(TransactionType.BASE);
-        TransactionType actual = transactionStatus.getTransactionType();
-        assertThat(actual, is(TransactionType.BASE));
-    }
-    
-    @Test
     void assertSetInTransaction() {
         transactionStatus.setInTransaction(true);
         boolean actual = transactionStatus.isInConnectionHeldTransaction();