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.h

@@ -13,7 +13,6 @@ public:
 	void init();
 
 	void play(int t);
-	void stopmusic();
 	void haltdasmusik();
 	void silencedasmusik();
 	void fadeMusicVolumeIn(int ms);