Revert "Forbid DROP SCHEMA on temporary namespaces"

This reverts commit a052f6cb, following complains from Robert Haas and
Tom Lane.  Backpatch down to 9.4, like the previous commit.

Discussion: https://postgr.es/m/CA+TgmobL4npEX5=E5h=5Jm_9mZun3MT39Kq2suJFVeamc9skSQ@mail.gmail.com
Backpatch-through: 9.4

src/backend/commands/dropcmds.c

@@ -101,21 +101,6 @@ RemoveObjects(DropStmt *stmt)
 						 errhint("Use DROP AGGREGATE to drop aggregate functions.")));
 		}
 
-		/*
-		 * Prevent the drop of a temporary schema, be it owned by the current
-		 * session or another backend as this would mess up with the callback
-		 * registered to clean up temporary objects at the end of a session.
-		 * Note also that the creation of any follow-up temporary object would
-		 * result in inconsistencies within the session whose temporary schema
-		 * has been dropped.
-		 */
-		if (stmt->removeType == OBJECT_SCHEMA &&
-			isAnyTempNamespace(address.objectId))
-			ereport(ERROR,
-					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
-					 errmsg("cannot drop temporary schema \"%s\"",
-							get_namespace_name(address.objectId))));
-
 		/* Check permissions. */
 		namespaceId = get_object_namespace(&address);
 		if (!OidIsValid(namespaceId) ||