multi_ProcessUtility shouldn't process postgres tables

Currently it seems that it is possible for a postgres table to be processed by `multi_ProcessUtility`. Ideally if a query doesn't contain a citus table, it should be processed with the `standard_ProcessUtility`.

When I was trying some new option with `VACUUM` for pg13 I have got an error on citus with a non-citus(postgres) table even though it should work fine with postgres.