When a map is deleted, remove it from the overmaparea too

[snipercup]

I came across this while working on something else. When a map is deleted, it will remove itself from the tacticalmap that it's in, but not from the overmaparea.

Make changes to Map.gd and it's delete function so that the overmaparea is updated as well.

Remove the references variable from DMap as well
Remove the add_reference_to_tile from dtiles as well