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.

This removes all languages besides en_US.

Change-Id: I56c85a63888d0edd81895fb213e5967b0e799013
Closes-Bug: #1317794
Merge "Remove all mostly untranslated PO files"