Get rid of old version of BuildTupleHashTable().

Get rid of old version of BuildTupleHashTable().

It was reasonable to preserve the old API of BuildTupleHashTable()
in the back branches, but in HEAD we should actively discourage use
of that version.  There are no remaining callers in core, so just
get rid of it.  Then rename BuildTupleHashTableExt() back to
BuildTupleHashTable().

While at it, fix up the miserably-poorly-maintained header comment
for BuildTupleHashTable[Ext].  It looks like more than one patch in
this area has had the opinion that updating comments is beneath them.

Discussion: https://postgr.es/m/538343.1734646986@sss.pgh.pa.us