x11: Drop unnecessary check to ignore crossing events

This check dates all the way back to commit ac2aa5337d. At the time, the
window switcher was an actual X window, that could generate crossing events
if popped up under the pointer. Checking for this kind of crossing events
made sense back at the time in order not to break focus-follows-mouse as
it's been behaving for long.

But now, this UI is all Clutter widgetry, which in the worst case (X11
sessions, of course) it will update the stage window shape to make these
parts clickable. This happens in other places of code that do already
check for ignoring crossing events.

Underneath, this looked up for a Mutter-local GdkWindow of type
GDK_WINDOW_TEMP, only the main MetaFrames window matches those characteristics
nowadays, notably no window switcher popups. Since the remaining window is
never unmapped (until perhaps shutdown), the paths were functionally dead.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2175>

src/x11/events.c

@@ -1934,18 +1934,6 @@ meta_x11_display_handle_xevent (MetaX11Display *x11_display,
 
   input_event = get_input_event (x11_display, event);
 
-  if (event->type == UnmapNotify)
-    {
-      if (meta_ui_window_should_not_cause_focus (x11_display->xdisplay,
-                                                 modified))
-        {
-          meta_display_add_ignored_crossing_serial (display, event->xany.serial);
-          meta_topic (META_DEBUG_FOCUS,
-                      "Adding EnterNotify serial %lu to ignored focus serials",
-                      event->xany.serial);
-        }
-    }
-
   if (handle_input_xevent (x11_display, input_event, event->xany.serial))
     {
       bypass_gtk = bypass_compositor = TRUE;