Remove Dispose() override

Culls another non-thread-safe mutation of the `Playing` bindable.
It seems to be a weird vestige from an earlier revision of the old
"direct" panel, which relied on `DisposeOnDeathRemoval` to finish track
playback (and then was removed in
6c150c9ed7). The play button is no longer
responsible for managing preview track lifetime anyway;
`PreviewTrackManager`'s method are intended for that.

osu.Game/Overlays/BeatmapListing/Panels/PlayButton.cs

@@ -163,11 +163,5 @@ namespace osu.Game.Overlays.BeatmapListing.Panels
             if (Preview?.Start() != true)
                 Playing.Value = false;
         }
-
-        protected override void Dispose(bool isDisposing)
-        {
-            base.Dispose(isDisposing);
-            Playing.Value = false;
-        }
     }
 }