Re: retire MemoryContextResetAndDeleteChildren backwards compatibility macro

I just found myself researching the difference between MemoryContextReset()
and MemoryContextResetAndDeleteChildren(), and it turns out that as of
commit eaa5808 (2015), there is none.
MemoryContextResetAndDeleteChildren() is just a backwards compatibility
macro for MemoryContextReset().  I found this surprising because it sounds
like they do very different things.

Shall we retire this backwards compatibility macro at this point?  A search
of https://codesearch.debian.net/ does reveal a few external uses, so we
could alternatively leave it around and just update Postgres to stop using
it, but I don't think it would be too burdensome for extension authors to
fix if we removed it completely.

Patch attached.