Codechange: Where the ship comes from is already known

This simplifies the handling of variables.

`ChooseShipTrack` is called upon entering `tile`, and looking further back to the caller, it can be deduced that `v->tile` matches `src_tile`. With that said, `enterdir` can also be removed, as it's not used anywhere else.

`CreateRandomPath` and `GetRandomFollowUpTrackdir` is being fed `src_tile` as it's 2nd parameter. This could be eliminated, as `v` is also being passed to it. Just use `v->tile` in those functions.

src/ship_cmd.cpp

@@ -501,14 +501,11 @@ static void ShipArrivesAt(const Vehicle *v, Station *st)
  *
  * @param v Ship to navigate
  * @param tile Tile, the ship is about to enter
- * @param enterdir Direction of entering
  * @param tracks Available track choices on \a tile
  * @return Track to choose, or INVALID_TRACK when to reverse.
  */
-static Track ChooseShipTrack(Ship *v, TileIndex tile, DiagDirection enterdir, TrackBits tracks)
+static Track ChooseShipTrack(Ship *v, TileIndex tile, TrackBits tracks)
 {
-	assert(IsValidDiagDirection(enterdir));
-
 	bool path_found = true;
 	Track track;
 
@@ -535,7 +532,7 @@ static Track ChooseShipTrack(Ship *v, TileIndex tile, DiagDirection enterdir, Tr
 
 		switch (_settings_game.pf.pathfinder_for_ships) {
 			case VPF_NPF: track = NPFShipChooseTrack(v, path_found); break;
-			case VPF_YAPF: track = YapfShipChooseTrack(v, tile, enterdir, path_found, v->path); break;
+			case VPF_YAPF: track = YapfShipChooseTrack(v, tile, path_found, v->path); break;
 			default: NOT_REACHED();
 		}
 	}
@@ -821,7 +818,7 @@ static void ShipController(Ship *v)
 				}
 
 				/* Choose a direction, and continue if we find one */
-				const Track track = ChooseShipTrack(v, gp.new_tile, diagdir, tracks);
+				const Track track = ChooseShipTrack(v, gp.new_tile, tracks);
 				if (track == INVALID_TRACK) return ReverseShip(v);
 
 				const ShipSubcoordData &b = _ship_subcoord[diagdir][track];