Remove duplicate function musicclass::stopmusic()

It is an exact duplicate of musicclass::haltdasmusik(), so use that
function instead and update callers. Looks like
musicclass::haltdasmusik() came first, anyway (musicclass::stopmusic()
was only used in editor.cpp).

desktop_version/src/Music.cpp

@@ -269,12 +269,6 @@ void musicclass::play(int t)
 	}
 }
 
-void musicclass::stopmusic()
-{
-	Mix_HaltMusic();
-	currentsong = -1;
-}
-
 void musicclass::haltdasmusik()
 {
 	Mix_HaltMusic();