Remove SetCoverTaskbarWhenMaximized from other backends.

src/Android/Avalonia.Android/Platform/SkiaPlatform/WindowImpl.cs

@@ -108,11 +108,6 @@ namespace Avalonia.Android.Platform.SkiaPlatform
         {
         }
 
-        public void SetCoverTaskbarWhenMaximized(bool enable)
-        {
-            //Not supported
-        }
-
         public void Invalidate(Rect rect)
         {
             if (Holder?.Surface?.IsValid == true) base.Invalidate();

src/Gtk/Avalonia.Gtk/WindowImplBase.cs

@@ -304,11 +304,6 @@ namespace Avalonia.Gtk
             args.RetVal = true;
         }
 
-        public void SetCoverTaskbarWhenMaximized(bool enable)
-        {
-            // No action neccesary on Gtk.
-        }
-
         public void Dispose()
         {
             _window.Hide();

src/iOS/Avalonia.iOS/AvaloniaView.cs

@@ -170,11 +170,6 @@ namespace Avalonia.iOS
             //Not supported
         }
 
-        public void SetCoverTaskbarWhenMaximized(bool enable)
-        {
-            //Not supported
-        }
-
         public override void TouchesEnded(NSSet touches, UIEvent evt)
         {
             var touch = touches.AnyObject as UITouch;