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/editor.cpp

@@ -3458,7 +3458,7 @@ void editorlogic()
         map.colstate = 10;
         game.gamestate = 1;
         graphics.fademode = 4;
-        music.stopmusic();
+        music.haltdasmusik();
         music.play(6);
         map.nexttowercolour();
         ed.settingsmod=false;
@@ -4398,7 +4398,7 @@ void editorinput()
                             game.edsavedir=1-edentity[testeditor].p1;
                         }
 
-                        music.stopmusic();
+                        music.haltdasmusik();
                         graphics.backgrounddrawn=false;
                         ed.returneditoralpha = 1000; // Let's start it higher than 255 since it gets clamped
                         script.startgamemode(21);