Makefile: remove a warning about old GETTEXT_POISON flag

Makefile: remove a warning about old GETTEXT_POISON flag

Remove a migratory warning I added in 6cdccfce1e0 (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 <redacted>
Signed-off-by: Junio C Hamano <redacted>