Remove all mostly untranslated PO files

We only import PO files that are at least 75 % translated (8 % in
common), so we can delete now all other PO files. The patch only
removes the mostly untranslated files. Once a file becomes mostly
translated, the bot will import it again.

A previous patch missed the user-guide and user-guide-admin locales,
remove files from there now as well. None of the files is sufficiently
translated yet.

Closes-Bug: #1317794

Change-Id: If531d2d262abc7f0e05686a0b8b2869954241c3d
Merge "Remove all mostly untranslated PO files"