Fix crash when quitting a QT app on the wayland backend using menu

QT unmaps the view before destroying the popup. We destroyed the popup
in response to the view unmapping, but then we'd attempt to destroy it a
second time which caused a crash.

The patch removes the listener.

I tested it with GTK as well, and can confirm the popup is still being
destroyed.

include/sway/tree/view.h

@@ -226,7 +226,6 @@ struct sway_view_child {
 	struct wl_listener surface_map;
 	struct wl_listener surface_unmap;
 	struct wl_listener surface_destroy;
-	struct wl_listener view_unmap;
 };
 
 struct sway_xdg_popup_v6 {

sway/tree/view.c

@@ -731,13 +731,6 @@ static void view_child_handle_surface_destroy(struct wl_listener *listener,
 	view_child_destroy(child);
 }
 
-static void view_child_handle_view_unmap(struct wl_listener *listener,
-		void *data) {
-	struct sway_view_child *child =
-		wl_container_of(listener, child, view_unmap);
-	view_child_destroy(child);
-}
-
 static void view_init_subsurfaces(struct sway_view *view,
 		struct wlr_surface *surface) {
 	struct wlr_subsurface *subsurface;
@@ -779,9 +772,6 @@ void view_child_init(struct sway_view_child *child,
 	child->surface_map.notify = view_child_handle_surface_map;
 	child->surface_unmap.notify = view_child_handle_surface_unmap;
 
-	wl_signal_add(&view->events.unmap, &child->view_unmap);
-	child->view_unmap.notify = view_child_handle_view_unmap;
-
 	struct sway_output *output = child->view->container->workspace->output;
 	wlr_surface_send_enter(child->surface, output->wlr_output);
 
@@ -791,7 +781,6 @@ void view_child_init(struct sway_view_child *child,
 void view_child_destroy(struct sway_view_child *child) {
 	wl_list_remove(&child->surface_commit.link);
 	wl_list_remove(&child->surface_destroy.link);
-	wl_list_remove(&child->view_unmap.link);
 
 	if (child->impl && child->impl->destroy) {
 		child->impl->destroy(child);