Remove duplicated italian locale

Now there are 2 italian translations:
* `it` (updated - 100%)
* `it_IT` (outdated - 10%, last update was in 2013)

The main issue is that almost 90% of italians (like me) have the system/browser language set to `it_IT`, so they will always see the outdated `it_IT` translation and not the updated `it` translation

Given that:
* We don't need multiple italian translations: Italian language in Italy (`it_IT`) and in Switzerland (`it_CH`) have no differences.
* the authors of `it_IT` appear already in the `it` .po file

I suggest to remove the `it_IT` translation from the repository and in transifex.