Fix editor potentially using a track post-disposal

This changes the editor to track the current track as it is *loaded* by
`MusicController`, rather than haphazardly following the current global
`WorkingBeatmap` (with a potentially unloaded track) or relying on local
immediate-load behaviour (as implemented in `ResourcesSection`).

osu.Game/Screens/Edit/Setup/ResourcesSection.cs

@@ -118,8 +118,6 @@ namespace osu.Game.Screens.Edit.Setup
             working.Value.Metadata.AudioFile = destination.Name;
 
             music.ReloadCurrentTrack();
-
-            editor?.UpdateClockSource();
             return true;
         }