Don't disallow DROP of constraints ONLY on partitioned tables

Don't disallow DROP of constraints ONLY on partitioned tables

This restriction seems to have come about due to some fuzzy thinking: in
commit 9139aa19423b we were adding a restriction against ADD constraint
ONLY on partitioned tables (which is sensible) and apparently we thought
the DROP case had to be symmetrical.  However, it isn't, and the
comments about it are mistaken about the effect it would have.  Remove
this limitation.

There have been no reports of users bothered by this limitation, so I'm
not backpatching it just yet.  We can revisit this decision later, as needed.

Reviewed-by: Amit Langote <[email protected]>
Discussion: https://postgr.es/m/202409261752[email protected]
Discussion: https://postgr.es/m/7682253a-6f79-6a92-00aa-267c4c412870@lab.ntt.co.jp
	(about commit 9139aa19423b, previously not registered)