Make Delete on macOS delete correctly.

On a Mac keyboard, the Delete key functions as the Backspace key
does on a PC keyboard.  Map VK_Delete to Key.Back (VK_ForwardDelete
correctly maps to Key.Delete)

src/OSX/Avalonia.MonoMac/KeyTransform.cs

@@ -200,7 +200,7 @@ namespace Avalonia.MonoMac
             [kVK_Return] = Key.Return,
             [kVK_Tab] = Key.Tab,
             [kVK_Space] = Key.Space,
-            [kVK_Delete] = Key.Delete,
+            [kVK_Delete] = Key.Back,
             [kVK_Escape] = Key.Escape,
             [kVK_Command] = Key.LWin,
             [kVK_Shift] = Key.LeftShift,