Clear tcbs.route_rsn when a route is set

This appears to fix the issue where route_rsn is not cleared and later
shown on the train HUD even when the route is set.

Note that this only works if the route is (successfully) set after this
patch is applied - it does not clear route_rsn for routes that are
already set.

How to test:
* Set a conflicting route for the train.
* Set a route for the train. Note that a message appears on the train
  HUD explaining that the route cannot be set (this is also the current
  behavior without the patch).
* Cancel the conflicting route. Note that the message on the train HUD
  is cleared.

advtrains_interlocking/routesetting.lua

@@ -113,6 +113,7 @@ function ilrs.set_route(signal, route, try)
 			if c_tcbs.signal then
 				c_tcbs.route_committed = true
 				c_tcbs.aspect = route.aspect or advtrains.interlocking.GENERIC_FREE
+				c_tcbs.route_rsn = nil
 				c_tcbs.route_origin = signal
 				advtrains.interlocking.update_signal_aspect(c_tcbs)
 			end