Do not call CDB_Invalidate_Varnish() if not owned by a superuser

Search for that function in schemas cartodb and public (in that order).
Closes #24

sql/test_ddl_triggers.sql

@@ -1,17 +1,5 @@
 \set VERBOSITY terse
 
--- Create a stub cdb_invalidate_varnish function if not available
---CREATE FUNCTION tmp() returns void AS $$
-DO $$
-BEGIN
-  PERFORM cdb_invalidate_varnish(0);
-EXCEPTION
-  WHEN undefined_function THEN
-    CREATE OR REPLACE FUNCTION cartodb.cdb_invalidate_varnish(tabname text)
-    RETURNS void AS '' LANGUAGE 'sql';
-END;
-$$ LANGUAGE 'plpgsql';
-
 -- Set user quota to infinite
 SELECT CDB_SetUserQuotaInBytes(0);