Fix memory leak while print missing column stats

Relation metadata reference was added twice due to which
memory leak is detected and PQO fallsback to planner. This patch
removes redundant AddRef for Relation Metadata and fixes fallback.
Signed-off-by: NEkta Khanna <ekhanna@pivotal.io>

src/backend/gpopt/utils/COptTasks.cpp

@@ -1204,7 +1204,6 @@ COptTasks::PrintMissingStatsWarning
 					oss << ", ";
 				}
 
-				pmdidRel->AddRef();
 				pmdidRel->AddRef();
 				phsmdidRel->FInsert(pmdidRel);
 				oss << pmdrel->Mdname().Pstr()->Wsz();