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