Allow deletion of junk translations

[Display Name]

There is currently no way for deleting junk translations that for some reason appear in the project. For example this one [1] appeared because I mistakenly associated some po filenames with that language during translation import. However, even after the po files were associated with their correct languages (and no po file were associated with this one anymore), this wrong translation continues to exist and continues to be exported into the target branch, so I have to delete it every time I perform a merge with Launchpad translations.

[1] https://translations.launchpad.net/pidgin++/trunk/+pots/pidgin/ab