Fix 3 CI errors: osuTK reference, unused field, and null check simplification

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/4b89ea94-0ef4-4f3a-9dd7-ffb7cd4e8d09

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game.Rulesets.Osu/Edit/OsuSliderVelocityToolboxGroup.cs

@@ -14,7 +14,7 @@
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Screens.Edit;
-using osuTK;
+using System.Numerics;
 
 namespace osu.Game.Rulesets.Osu.Edit
 {

osu.Game/Screens/Edit/Components/FormSampleSet.cs

@@ -274,8 +274,7 @@ private void updateState()
 
                 recycleSamples();
 
-                if (triangles != null)
-                    triangles.Colour = ColourInfo.GradientVertical(triangleGradientSecondColour ?? default, BackgroundColour);
+                triangles?.Colour = ColourInfo.GradientVertical(triangleGradientSecondColour ?? default, BackgroundColour);
             }
 
             private void recycleSamples() => Schedule(() =>

osu.Game/Screens/OnlinePlay/Matchmaking/Queue/ScreenQueue.cs

@@ -440,8 +440,6 @@ private void onMatchmakingLobbyStatusChanged(MatchmakingLobbyStatus status) => S
             loadRecentMatches(status.RecentMatches.OfType<RankedPlayRoomState>().ToArray()).FireAndForget();
         });
 
-        private int historyInsertOrder;
-
         private async Task loadRecentMatches(RankedPlayRoomState[] matches)
         {
             // matches initial API response.