Remove manual override for add section text

This now automatically picks the message if it is defined.
skin-action-addsection isn't defined yet, so the fallback uses
action-addsection, which is shared with permissions.
mirage-action-addsection can be removed once the new message is
defined and translated.

Ref: T301203
Change-Id: I3d5ed7c81be3791506d8a6dee19ef933c9b559fb