it removes, and decrements the available slot count for every node
in a pclass that is being removed.

After running prune_unusable_pclasses(), we now do another type check,
minus the pruned pclasses.

This should enable us to detect more free node problems up front -
let's say there are 8 vnodes that all want 2 interfaces, but only
a couple pnodes that have 2: most have 1. The second type check will
now catch this, since all of the 1-interface pnodes have been pruned.