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: Ia2953a0e78afc4b440f3a42363184aad629c9434
Closes-Bug: #1317794