Remove check from relcache invalidation callback

The timescaledb extension checking code uses syscache lookups to
determine whether the proxy table exists. Doing syscache lookups
inside the invalidation callback context was corrupting the syscache.
For this reason, we remove this callback.

src/loader/loader.c

@@ -125,14 +125,6 @@ ts_loader_extension_exists(void)
 	return extension_exists();
 }
 
-static void
-inval_cache_callback(Datum arg, Oid relid)
-{
-	if (guc_disable_load)
-		return;
-	extension_check();
-}
-
 static bool
 drop_statement_drops_extension(DropStmt *stmt)
 {
@@ -573,7 +565,6 @@ _PG_init(void)
 	 * cannot check for extension here since not inside a transaction yet. Nor
 	 * do we even have an assigned database yet
 	 */
-	CacheRegisterRelcacheCallback(inval_cache_callback, PointerGetDatum(NULL));
 
 	/*
 	 * using the post_parse_analyze_hook since it's the earliest available