Fix memory leak in GpPolicyFetch() called in RelationBuildDesc() 
Fix memory leak in GpPolicyFetch() called in RelationBuildDesc() 

Fix memory leak in GpPolicyFetch() called in RelationBuildDesc() when the table is
an external table. This commit fixes github issue https://github.com/greenplum-db/gpdb/issues/11244.
Also, remove the Assert(CurrentMemoryContext != CacheMemoryContext).  I still think
we should add this assert, but the reality is that there are many places in the code that
overuse the CacheMemoryContext, so we have to remove this assert.

adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb