G1: G1CollectedHeap should inherit directly from CollectedHeap

Currently, G1CollectedHeap inherits from SharedHeap. The reason for this was probably that it got the perm gen from SharedHeap "for free". However, after the perm gen has been removed there's probably no big benefit from inheriting from SharedHeap (G1 doesn't really share much with the GenCollectedHeap GCs). So, we should make G1CH inherit directly from CollectedHeap instead.