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.

web/templates/settings_journey.html

@@ -13,8 +13,8 @@
   Next connections for a fixed route -- your daily commute, say. Uses
   regional EFA transit-planner backends (not Deutsche Bahn's own
   infrastructure, which has been actively blocking third-party access
-  lately) -- several are tried in order, so this can work even when the
-  Departures screen's DB-based source can't.
+  lately) -- several are tried in order, so this stays reliable even when
+  one regional backend is down.
 </p>
 
 <form method="post">