screens: Remove explicit Screen constructors

This constructor didn't do anything since DisplayApp reference was
removed from the Screen base class.

src/displayapp/screens/Navigation.cpp

@@ -130,7 +130,7 @@ namespace {
  *
  */
 Navigation::Navigation(Pinetime::Applications::DisplayApp* app, Pinetime::Controllers::NavigationService& nav)
-  : Screen(app), navService(nav) {
+  : navService(nav) {
 
   imgFlag = lv_label_create(lv_scr_act(), nullptr);
   lv_obj_set_style_local_text_font(imgFlag, LV_LABEL_PART_MAIN, LV_STATE_DEFAULT, &lv_font_navi_80);