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/journey.py

@@ -3,9 +3,8 @@ regional EFA (Elektronische Fahrplanauskunft) instances -- transit-planner
 backends run by regional transit authorities, not Deutsche Bahn itself.
 
 This deliberately avoids bahn.de's own internal trip-planner API. DB has
-been actively blocking third-party access to its own backends recently (see
-the Departures screen's docstring and the outage that motivated it) -- EFA
-instances aren't DB's infrastructure, so they aren't part of that
+been actively blocking third-party access to its own backends recently --
+EFA instances aren't DB's infrastructure, so they aren't part of that
 clampdown. Several are tried in sequence (not parallel, to keep this
 synchronous like the rest of the app); first one to return usable journeys
 wins. Ported and simplified from github.com/EiS94/db_info (a Home Assistant