display: Ignore the modmask passed into meta_display_begin_grab_op

A careful analysis of mutter's codebase shows that nothing actually
passes anything but 0 to this. gnome-shell has one instance, but it's
most likely a mistake.

Remove the grab_mask field and the one place in keybindings.c that uses it.

The parameter to begin_grab_op is left in for API compatibility reasons.

src/core/keybindings.c

@@ -1763,7 +1763,7 @@ process_overlay_key (MetaDisplay *display,
 
           binding = display_get_keybinding (display,
                                             display->overlay_key_combo.keycode,
-                                            display->grab_mask);
+                                            0);
           if (binding &&
               meta_compositor_filter_keybinding (display->compositor, binding))
             return TRUE;