pgsql: Remove pg_dump/pg_dumpall support for dumping from pre-9.2 serve

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/30e7c175b81d53c0f60f6ad12d1913a6d7d77008

Modified Files
--------------
doc/src/sgml/ref/pg_dump.sgml        |   19 +-
src/bin/pg_dump/dumputils.c          |   32 +-
src/bin/pg_dump/pg_backup_archiver.c |    9 +-
src/bin/pg_dump/pg_dump.c            | 1846 +++++++---------------------------
src/bin/pg_dump/pg_dumpall.c         |  272 +----
5 files changed, 417 insertions(+), 1761 deletions(-)