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/base.html

@@ -189,7 +189,6 @@
       <a href="{{ url_for('index') }}" class="{{ 'active' if request.endpoint == 'index' }}">Status</a>
       <a href="{{ url_for('settings_navidrome') }}" class="{{ 'active' if request.endpoint == 'settings_navidrome' }}">Navidrome</a>
       <a href="{{ url_for('settings_weather') }}" class="{{ 'active' if request.endpoint == 'settings_weather' }}">Weather</a>
-      <a href="{{ url_for('settings_departures') }}" class="{{ 'active' if request.endpoint == 'settings_departures' }}">Departures</a>
       <a href="{{ url_for('settings_journey') }}" class="{{ 'active' if request.endpoint == 'settings_journey' }}">Journey</a>
       <a href="{{ url_for('settings_picture') }}" class="{{ 'active' if request.endpoint == 'settings_picture' }}">Picture</a>
       <a href="{{ url_for('settings_rotation') }}" class="{{ 'active' if request.endpoint == 'settings_rotation' }}">Rotation</a>