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.

Change-Id: Iaea7a29267ee554d0d20b5cd050d453cea3798ba
Closes-Bug: #1317794
Merge "Remove all mostly untranslated PO files"