Remove pg_dump/pg_dumpall support for dumping from pre-9.2 servers.

Remove pg_dump/pg_dumpall support for dumping from pre-9.2 servers.

Per discussion, we'll limit support for old servers to those branches
that can still be built easily on modern platforms, which as of now
is 9.2 and up.  Remove over a thousand lines of code dedicated to
dumping from older server versions.  (As in previous changes of
this sort, we aren't removing pg_restore's ability to read older
archive files ... though it's fair to wonder how that might be
tested nowadays.)  This cleans up some dead code left behind by
commit 989596152.

Discussion: https://postgr.es/m/2923349.1634942313@sss.pgh.pa.us