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.

README.md

@@ -1,8 +1,8 @@
 # Paperwall
 
 A standalone dashboard for one Waveshare `epd4in0e` (4", 6-color) e-ink panel:
-Navidrome now-playing, clock, weather, departure boards, a fixed-route
-journey planner, and pictures (upload or URL), switchable/controllable from
+Navidrome now-playing, clock, weather, a fixed-route journey planner, and
+pictures (upload or URL), switchable/controllable from
 a local web UI. Started as an InkyPi replacement, but doesn't depend on
 InkyPi at all -- own venv, own vendored copy of the Waveshare driver files.