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
------
REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/77971bc4f2371ce25b87b105470a39d5f21815d1

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