core: Simplify handling of ungrabbing condition

Unlike the comment suggests, this piece of event handling manages
the ungrabbing of a window on button press in the following 2
conditions:

- If a keyboard grab operation was triggered, the window does
  additionally follow the pointer, and first button press ends
  the grab.

- If a button-press grab is ongoing on the window, but more buttons
  are pressed.

We can simplify this to just happen every time a button press event
is received while a window grab op is ongoing. The only case where
this might diverge a bit is same button presses from different
pointer devices, and it's not a big stretch to also undo the grab
in that situation.

This also happens to make the "button" argument in
meta_display_begin_grab_op() completely unused.

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

src/core/window.c

@@ -6300,22 +6300,14 @@ meta_window_handle_mouse_grab_op_event  (MetaWindow         *window,
 
     case CLUTTER_BUTTON_PRESS:
       {
-        ClutterModifierType grab_mods = meta_display_get_compositor_modifiers (window->display);
-
         /* This is the keybinding or menu case where we've
          * been dragging around the window without the button
-         * pressed. */
-
-        if ((meta_grab_op_is_mouse (window->display->grab_op) &&
-             (event->button.modifier_state & grab_mods) == grab_mods &&
-             window->display->grab_button != (int) event->button.button) ||
-            meta_grab_op_is_keyboard (window->display->grab_op))
-          {
+         * pressed, or the case of pressing extra mouse buttons
+         * while a grab op is ongoing.
+         */
         end_grab_op (window, event);
         return FALSE;
       }
-        return TRUE;
-      }
 
     case CLUTTER_TOUCH_END:
       if (!meta_display_is_pointer_emulating_sequence (window->display, sequence))