Remove untranslated PO files

The proposal bot only imports file that are at least 75 % translated.
Remove any PO file that is untranslated or has less than 75 % of
translations.

Checked number of translated entries with:

for i in `find . -name *.po `; do msgfmt --statistics -v $i; done

Whenever a file is again translated sufficiently, the proposal bot will
propose a new patch - thus we do not lose any translations.

Change-Id: I30e4849464bc8ace8796c699a094a4dfcc140b50