pgsql: pg_upgrade: remove pre-8.4 code and >= 8.4 check

pg_upgrade: remove pre-8.4 code and >= 8.4 check

We only support upgrading from >= 8.4 so no need for this code or tests.

Reported-by: Magnus Hagander

Discussion: /messages/by-id/CABUevEx-D0PNVe00tkeQRGennZQwDtBJn=493MJt-x6sppbUxA@mail.gmail.com

Backpatch-through: 9.5

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/700b7002f2727cd1f9e01c5def6023b78017b958

Modified Files
--------------
src/bin/pg_upgrade/check.c | 29 +++++++----------------------
src/bin/pg_upgrade/relfilenode.c | 16 ++++++----------
2 files changed, 13 insertions(+), 32 deletions(-)