UnmarkObjectDistributed calls in utility hook may be obsolete

For various DROP commands (e.g. DROP TYPE, DROP COLLATION, DROP SERVER), we have logic to extract the distributed objects from the statement and call UnmarkObjectDistributed.

However, we already call master_unmark_object_distributed via citus_drop_trigger, which also calls UnmarkObjectDistributed from the standard process utility function. That is more correct, since that also works in scenarios like DROP SCHEMA .. CASCADE.

Hence, the UnmarkObjectDistributed calls in the utility hook seem obsolete.