G1 G1AllocRegion maintains a _used_bytes_before which is only used by the G1GCAllocRegion child class. Move _used_bytes_before out of base class.
Also remove unnecessary wrapper methods such as `HeapWord* allocate(HeapRegion* alloc_region, size_t word_size)`.
Also remove unnecessary wrapper methods such as `HeapWord* allocate(HeapRegion* alloc_region, size_t word_size)`.