library: don't remove the source if the user has changed his mind

@MartijnKaijser made me aware of the fact that when a user chooses "Remove source" on a music source but then says "No" in the "Are you sure?" dialog it still asks whether the removed source should be cleaned. After looking into it I noticed that the same problem existed for video sources. Even worse it actually removes the source independent of whether you choose "Yes" or "No".

Hint: use `?w=1` for easier review.