I18n: Remove broken / unsupported translations.

See http://users.xfce.org/~nick/broken-i18n/ for files and logs.

Use https://www.transifex.com/projects/p/xfce/ to upload fixed versions or request a translation team.
Remove unneeded files.
I18n: Add new translation sr (100%).

7 translated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).
I18n: Add new translation en_AU (100%).

7 translated messages.

Transifex (https://www.transifex.com/projects/p/xfce/).