Simplify `xfce4-about/translators.h` file

This file is a pain to maintain, syntax is a mess, is not updated regularly, some people want to be in the list, other don't want their email listed , and transifex does not provides stats.

I propose:

• remove mail address from this file
• remove the is_coordinator flag (never used)

Maybe:

• Re-add the txcron feature to fullfill a DB on every translation commit, with an easier way to update xfce4-about/translators.h from this DB on every major release, and document the process. The translators.h is hard to update in this form btw.

or

• Remove the translators list, because we can't keep it updated and is source of confusion

Also, find a way to update https://www.xfce.org/about/credits , the translator list has not been updated since 4.12