NavWidget: standardize back callback (#37479)

clean this up

system/ui/widgets/nav_widget.py

@@ -55,7 +55,6 @@ class NavWidget(Widget, abc.ABC):
 
   def __init__(self):
     super().__init__()
-    self._back_callback: Callable[[], None] | None = None
     self._back_button_start_pos: MousePos | None = None
     self._swiping_away = False  # currently swiping away
     self._can_swipe_away = True  # swipe away is blocked after certain horizontal movement
@@ -76,9 +75,6 @@ class NavWidget(Widget, abc.ABC):
   def set_back_enabled(self, enabled: bool | Callable[[], bool]) -> None:
     self._back_enabled = enabled
 
-  def set_back_callback(self, callback: Callable[[], None]) -> None:
-    self._back_callback = callback
-
   def _handle_mouse_event(self, mouse_event: MouseEvent) -> None:
     # FIXME: disabling this widget on new push_widget still causes this widget to track mouse events without mouse down
     super()._handle_mouse_event(mouse_event)
@@ -167,8 +163,7 @@ class NavWidget(Widget, abc.ABC):
       new_y = self._pos_filter.x = 0.0
 
     if new_y > self._rect.height + DISMISS_PUSH_OFFSET - 10:
-      if self._back_callback is not None:
-        self._back_callback()
+      gui_app.pop_widget()
 
       self._playing_dismiss_animation = False
       self._back_button_start_pos = None