Don't show "Map" in menu when configured to use external map

The map option is only useful when using an internal map that includes room information.

Before:
![iosched_with_map_menu_item](https://cloud.githubusercontent.com/assets/218061/4911421/4ea34b22-648d-11e4-8506-ef50022af0b7.png)

After:
![iosched_without_map_menu_item](https://cloud.githubusercontent.com/assets/218061/4911423/53d85808-648d-11e4-9d04-8a613bb8b071.png)