Remove Assert that matview is not in system schema from REFRESH.

We don't want to prevent an extension which creates a matview from
being installed in pg_catalog.

Issue was raised by Hitoshi Harada.
Backpatched to 9.3.

src/backend/commands/matview.c

@@ -178,9 +178,6 @@ ExecRefreshMatView(RefreshMatViewStmt *stmt, const char *queryString,
 				(errcode(ERRCODE_SYNTAX_ERROR),
 				 errmsg("CONCURRENTLY and WITH NO DATA options cannot be used together")));
 
-	/* We're not using materialized views in the system catalogs. */
-	Assert(!IsSystemRelation(matviewRel));
-
 	/* We don't allow an oid column for a materialized view. */
 	Assert(!matviewRel->rd_rel->relhasoids);