Remove AngularSeparation as a distinct class

As discussed in #902, the need for a separate `AngularSeparation` class is less obvious once `Angle` becomes a subclass of `Quantity` (#1006).

The main possible reason to keep it is that it currently disallows all operations, while `Angle` allows addition/subtraction with other angles and scaling by a scalar.  This is useful mainly because  `AngularSeparation` is intrinsically a single angle, but users may be tempted to think it's an on-sky *vector* and hence that they can add it to other separations or coordinates and get something more than just the scalar sum of the angle values.

I'm not strongly attached to keeping it, so that probably means it should be removed once the appropriate prerequisites are in, but others may have stronger opinions (@cdeil @adrn @demitri ?)