Remove Scheduler.AddOnce from updateSpecifics

To keep things simple, let's not bother debouncing this. The debouncing
was causing spectating handling to fail because of two interdependent
components binding to `BeatmapAvailability`:

Binding to update the screen's `Beatmap` after a download completes:

https://github.com/ppy/osu/blob/58747061171c4ebe70201dfe4d3329ed7f4343f5/osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs#L266-L267

Binding to attempt a load request:

https://github.com/ppy/osu/blob/8bb7bea04e56fab9247baa59ae879e16c8b4bd9b/osu.Game/Screens/OnlinePlay/Multiplayer/MultiplayerMatchSubScreen.cs#L67

The first must update the beatmap before the second runs, else gameplay
will not load due to `Beatmap.IsDefault`.

Author: peppy

osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs

@@ -427,7 +427,7 @@ protected void StartPlay()
         /// <returns>The screen to enter.</returns>
         protected abstract Screen CreateGameplayScreen(PlaylistItem selectedItem);
 
-        private void updateSpecifics() => Scheduler.AddOnce(() =>
+        private void updateSpecifics()
         {
             if (!this.IsCurrentScreen() || SelectedItem.Value is not PlaylistItem item)
                 return;
@@ -487,7 +487,7 @@ private void updateSpecifics() => Scheduler.AddOnce(() =>
             }
             else
                 UserStyleSection.Hide();
-        });
+        }
 
         protected virtual APIMod[] GetGameplayMods() => UserMods.Value.Select(m => new APIMod(m)).Concat(SelectedItem.Value!.RequiredMods).ToArray();