Makefile: remove a warning about old GETTEXT_POISON flag

Remove a migratory warning I added in 6cdccfce1e (i18n: make
GETTEXT_POISON a runtime option, 2018-11-08) to give anyone using that
option in their builds a heads-up about the change from compile-time
to runtime introduced in that commit.

It's been more than 2 years since then, anyone who ran into this is
likely to have made a change as a result, so removing this is long
overdue.

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

Makefile

@@ -1554,9 +1554,6 @@ endif
 ifdef NO_SYMLINK_HEAD
 	BASIC_CFLAGS += -DNO_SYMLINK_HEAD
 endif
-ifdef GETTEXT_POISON
-$(warning The GETTEXT_POISON option has been removed in favor of runtime GIT_TEST_GETTEXT_POISON. See t/README!)
-endif
 ifdef NO_GETTEXT
 	BASIC_CFLAGS += -DNO_GETTEXT
 	USE_GETTEXT_SCHEME ?= fallthrough