Revert "Ava UI: Input Menu Refactor (#4998)"

This reverts commit 49b37550ca.

This currently breaks the GTK GUI.

src/Ryujinx.Input/Assigner/IButtonAssigner.cs

@@ -31,6 +31,6 @@ namespace Ryujinx.Input.Assigner
         /// Get the pressed button that was read in <see cref="ReadInput"/> by the button assigner.
         /// </summary>
         /// <returns>The pressed button that was read</returns>
-        ButtonValue? GetPressedButton();
+        string GetPressedButton();
     }
 }