Remove mention of clock being nullable in IBeatSyncProvider

Co-authored-by: Susko3 <Susko3@protonmail.com>

osu.Game/Beatmaps/IBeatSyncProvider.cs

@@ -22,7 +22,7 @@ namespace osu.Game.Beatmaps
         ControlPointInfo? ControlPoints { get; }
 
         /// <summary>
-        /// Access a clock currently responsible for providing beat sync. If <c>null</c>, no current provider is available.
+        /// Access a clock currently responsible for providing beat sync.
         /// </summary>
         IClock Clock { get; }
     }