tests: remove uses of GIT_TEST_GETTEXT_POISON=false

tests: remove uses of GIT_TEST_GETTEXT_POISON=false

As noted in previous commits we are removing the use of
GIT_TEST_GETTEXT_POISON=false. These tests all relied on the facility
being off, it always is off after an earlier change, but we hadn't
removed the redundant assignments to "false" in the tests.

I'm preserving the deletion of "error" lines in 38b9197a76a (t5411:
add basic test cases for proc-receive hook, 2020-08-27), it turns out
that's useful even without GIT_TEST_GETTEXT_POISON=true in
play. Update a comment added in that commit to note that.

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