Merge branch 'ab/detox-gettext-tests'

Get rid of "GETTEXT_POISON" support altogether, which may or may
not be controversial.

* ab/detox-gettext-tests:
  tests: remove uses of GIT_TEST_GETTEXT_POISON=false
  tests: remove support for GIT_TEST_GETTEXT_POISON
  ci: remove GETTEXT_POISON jobs

ci/install-dependencies.sh

@@ -72,7 +72,7 @@ Documentation)
 	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor
 	;;
-linux-gcc-4.8|GETTEXT_POISON)
+linux-gcc-default|linux-gcc-4.8)
 	sudo apt-get -q update
 	sudo apt-get -q -y install $UBUNTU_COMMON_PKGS
 	;;