remove route_calculation setting, always use OSRM routing (#1064)

The per-user route_calculation toggle was a second, hidden on/off layer
on top of the day footer's show-route button, and made it easy to end up
with straight-line routes for no obvious reason. Drop the setting
entirely: routing is always on, the footer toggle stays the single
switch. Old stored values are simply ignored (settings are key-value, no
migration needed).

client/src/pages/DashboardPage.test.tsx

@@ -857,7 +857,6 @@ describe('DashboardPage', () => {
           temperature_unit: 'fahrenheit',
           time_format: '12h',
           show_place_description: false,
-          route_calculation: false,
           blur_booking_codes: false,
           dashboard_currency: 'on',
           dashboard_timezone: 'on',