t7415: remove out-dated comment about translation

t7415: remove out-dated comment about translation

Since GETTEXT_POISON does not exist anymore, there is no point warning
people about whether we should use test_i18ngrep. This is doubly
confusing because the comment was describing why it was OK to use grep,
but it got caught up in the mass conversion of 674ba34038 (fsck: mark
strings for translation, 2018-11-10).

Note there are other uses of test_i18ngrep in this script which are now
obsolete; I'll save those for a mass-cleanup. My goal here was just to
fix the confusing comment in code I'm about to refactor.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>