Cleanup: [NewGRF] Callback 14 used a weird value for var10 in the GUI.

The value 0x2110000 probably originated from a mixup between callback 14 (sprite layout) and 24 (tile layout).
The latter does indeed use a var10 value like that.

src/newgrf_station.cpp

@@ -788,7 +788,7 @@ bool DrawStationTile(int x, int y, RailType railtype, Axis axis, StationClassID
 	if (statspec == nullptr) return false;
 
 	if (HasBit(statspec->callback_mask, CBM_STATION_SPRITE_LAYOUT)) {
-		uint16 callback = GetStationCallback(CBID_STATION_SPRITE_LAYOUT, 0x2110000, 0, statspec, nullptr, INVALID_TILE);
+		uint16 callback = GetStationCallback(CBID_STATION_SPRITE_LAYOUT, 0, 0, statspec, nullptr, INVALID_TILE);
 		if (callback != CALLBACK_FAILED) tile = callback;
 	}