Remove the Departures screen

Not needed any more -- the Journey screen already covers the same
regional-transit need via EFA backends, and Departures depended on
the free db-vendo-client API that DB has been actively blocking third
parties from anyway.

Removed screens/departures.py, its settings template, its route, its
registration in screens/__init__.py, and its nav link. Reworded the
docstring/hint references in journey.py and settings_journey.html
that pointed at the now-gone screen. Existing config.json entries
referencing "departures" (rotation list, pinned screen) are harmless
leftovers -- both scheduler lookups already filter to screens that
still exist, so nothing needs a migration step.

Verified: app boots clean, /settings/departures 404s, every other
settings page still loads fine, no leftover references outside of
journey.py's own unrelated "departure time" wording.

screens/__init__.py

@@ -1,5 +1,4 @@
 from screens.clock import ClockScreen
-from screens.departures import DeparturesScreen
 from screens.journey import JourneyScreen
 from screens.navidrome import NavidromeScreen
 from screens.picture import PictureScreen
@@ -8,7 +7,6 @@ from screens.weather import WeatherScreen
 SCREEN_CLASSES = {
     ClockScreen.id: ClockScreen,
     WeatherScreen.id: WeatherScreen,
-    DeparturesScreen.id: DeparturesScreen,
     JourneyScreen.id: JourneyScreen,
     PictureScreen.id: PictureScreen,
     NavidromeScreen.id: NavidromeScreen,