prune_unusable_pclasses() now goes through the ptypes of the classes
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.
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