Remove ambiguous entries in Italian translation

The Italian translation is using "cancella" to translate various English terms like "cancel", "delete", "clear" o "un-something", this is confusing if some of these terms are used near each other (e.g. two buttons "cancel" and "delete").

See issue https://github.com/github/android/issues/498
Merge pull request #499 from kappa7194/patch-1

Remove ambiguous entries in Italian translation