Fix depth of header background.

Thanks to the order of adding to parent container, the depth value is actually not needed.

osu.Game/Overlays/SettingsOverlay.cs

@@ -162,8 +162,7 @@ namespace osu.Game.Overlays
                 Add(headerBackground = new Box
                 {
                     Colour = Color4.Black,
-                    RelativeSizeAxes = Axes.X,
-                    Depth = float.MaxValue
+                    RelativeSizeAxes = Axes.X
                 });
             }