ci: remove GETTEXT_POISON jobs

ci: remove GETTEXT_POISON jobs

A subsequent commit will remove GETTEXT_POISON entirely, let's start
by removing the CI jobs that enable the option.

We cannot just remove the job because the CI is implicitly depending
on the "poison" job being a sort of "default" job in the sense that
it's the job that was otherwise run with the default compiler, no
other GIT_TEST_* options etc. So let's keep it under the name
"linux-gcc-default".

This means we can remove the initial "make test" from the "linux-gcc"
job (it does another one after setting a bunch of GIT_TEST_*
variables).

I'm not doing that because it would conflict with the in-flight
334afbc76fb (tests: mark tests relying on the current default for
`init.defaultBranch`, 2020-11-18) (currently on the "seen" branch, so
the SHA-1 will almost definitely change). It's going to use that "make
test" again for different reasons, so let's preserve it for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>