Cleanups in Generation related code

Generation::spec() is only called in a few places and all of them are really looking for the initial size of the generation. Generation::spec() should be replaced with Generation::initial_size().

GenCollectedHeap::gc_stats() is unused and pointless. It should be removed.

Generation::full_collects_young_generation() is only ever called in old generations, and all old generations override it and return !ScavengeBeforeFullGC. The method should be removed and the flag checked directly in the only place where the method is called.