remove map_x continent from schema
The map_x_continent relation was a problem for relational integrity.
The continent that a territory belongs to is an attribute of a
territory, and so it is possible to simply perform a more sophisticated
query of the map_x_territory table (joining as appropriate) in order
to determine the continents that exist for a given map. Prior to this
change, it was possible for there to be continents associated with a
map, but no territories assigned to those continents. It was also
possible for territories to be assigned to continents, but to a
continent not represented in map_x_continent. This change corrects a
fundamental design flaw in the database that occurred due to a
combination of too many beers and jetlag.