Remove all mostly untranslated PO files

We only import PO files that are at least 75 % translated,
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.

Change-Id: I9b0eaa3d137c3728dc95d1559debae1ba57974f2
Closes-Bug: #1317794