Remove duplicated condition in primary key check (#1240)

cassandra/cqlengine/management.py

diff --git a/cassandra/cqlengine/management.py b/cassandra/cqlengine/management.py
index 7ca30f9..b34b003 100644
--- a/cassandra/cqlengine/management.py
+++ b/cassandra/cqlengine/management.py
@@ -256,7 +256,7 @@
 
                 continue
 
-            if col.primary_key or col.primary_key:
+            if col.primary_key:
                 msg = format_log_context("Cannot add primary key '{0}' (with db_field '{1}') to existing table {2}", keyspace=ks_name, connection=connection)
                 raise CQLEngineException(msg.format(model_name, db_name, cf_name))