Remove the T2DMap default constructor.

T2DMap currently has two default constructors and neither of them are
used. So, remove both.
The parent parameter is always passed in as currently used and I'm not familiar enough with Qt to really understand what is implied when it's not null and whether that's reasonable in this case or not. But, one can always pass a null pointer explicitly anyways, if that is desired. It doesn't need to be a default constructor.