core: Drop unused arguments to meta_display_grab_op()

Now that it is called from a single place, there's a few arguments
that are unnecessary:

- button and modifiers are unused
- already_grabbed was originally added to handle grab transitions between
  window menus (GtkMenus, back in the day) with display grabs. It's no
  longer necessary now
- frame_action can be passed through the META_GRAB_OP_WINDOW_FLAG UNCONSTRAINED
  flag

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2683>

src/core/window.c

@@ -6756,13 +6756,12 @@ meta_window_begin_grab_op (MetaWindow *window,
       y = pos.y;
     }
 
+  if (!frame_action)
+    op |= META_GRAB_OP_WINDOW_FLAG_UNCONSTRAINED;
+
   return meta_display_begin_grab_op (window->display,
                                      window,
                                      op,
-                                     FALSE,
-                                     frame_action,
-                                     0 /* button */,
-                                     0,
                                      timestamp,
                                      x, y);
 }