Support Removal of Subclasses

[NellsRelo]

We can currently add subclasses to a Progression's Subclass node, but it may be useful to be able to remove Subclasses as well. If, for example, a total conversion or massive overhaul wants to replace a vanilla subclass with its own, and also avoid being affected by mods that do things to the original Subclass, being able to remove subclasses that you don't want the user to select could be useful.

Another use-case would be for if you wanted to change the level at which subclasses are made available to a class.