git-gui: treat the message template file as a built file

git-gui: treat the message template file as a built file

Follow the lead of 5377abc0c9d5 ("po/git.pot: don't check in result
of "make pot"", 2022-05-26) in the Git repository and do not track
git-gui.pot anymore.

Instead, translators are expected to integrate an up-to-date version
from the master branch into their translation file using

   make ALL_POFILES=po/xx.po update-po

Update README to describe the new process. It is now understood that
different translations need not be based on the same message template
file, but rather individual translators should base their translation
on the most up-to-date code. Remove the section that addresses the
i18n coordinator as it does not apply when no common base is required
among translators.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>