test-lib: remove check_var_migration

test-lib: remove check_var_migration

Remove the check_var_migration() migration helper. This was added back
in [1], [2] and [3] to warn users to migrate from e.g. the
"GIT_FSMONITOR_TEST" name to "GIT_TEST_FSMONITOR".

I daresay that having been warning about this since late 2018 (or
v2.20.0) was sufficient time to give everyone interested a heads-up
about moving to the new names.

I don't see the need for going through the "do this later" codepath
anticipated in [1], let's just remove this instead.

1. 4cb54d0aa8e (fsmonitor: update GIT_TEST_FSMONITOR support,
   2018-09-18)
2. 1f357b045b5 (read-cache: update TEST_GIT_INDEX_VERSION support,
   2018-09-18)
3. 5765d97b71d (preload-index: update GIT_FORCE_PRELOAD_TEST support,
   2018-09-18)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>