po/git.pot: don't check in result of "make pot"

Remove the "po/git.pot" file from being tracked, which started with
dce37b66fb0 (l10n: initial git.pot for 1.7.10 upcoming release,
2012-02-13).

The reason the po/git.pot started being checked in was because the
po/*.po files were changed a schema where we'd generate them from a
known-good snapshot of po/git.pot, instead of each translator running
"make pot" themselves.

This makes sense, but we don't need to carry this file in-tree just to
achieve that aim, and doing so has resulted in a significant amount of
"diff churn" since this method of doing it was introduced:

$ git log -p --oneline -- po/git.pot|wc -l
553743

We can instead let l10n contributors to generate "po/git.pot" in runtime
to update their own "po/XX.po", and the l10n coordinator can check
pull requests using CI pipeline.

This reverts to the schema introduced initially in cd5513a7168 (i18n:
Makefile: "pot" target to extract messages marked for translation,
2011-02-22).

The actual "git rm" of po/git.pot was in preceding commit to make this
change easier to review, and to preempt the mailing list from blocking
it due to it being too large.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>