NIFIREG-426 Remove MySQL 5.6 from test-all-dbs as it is no longer supported by the latest Flyway version (#310)

diff --git a/nifi-registry-core/pom.xml b/nifi-registry-core/pom.xml
index b373867..1290716 100644
--- a/nifi-registry-core/pom.xml
+++ b/nifi-registry-core/pom.xml
@@ -172,19 +172,6 @@
                         <artifactId>maven-failsafe-plugin</artifactId>
                         <executions>
                             <execution>
-                                <id>mysql5.6-test</id>
-                                <phase>verify</phase>
-                                <goals>
-                                    <goal>integration-test</goal>
-                                    <goal>verify</goal>
-                                </goals>
-                                <configuration>
-                                    <systemPropertyVariables>
-                                        <spring.profiles.active>mysql-56</spring.profiles.active>
-                                    </systemPropertyVariables>
-                                </configuration>
-                            </execution>
-                            <execution>
                                 <id>mysql5.7-test</id>
                                 <phase>verify</phase>
                                 <goals>