Remove G1CollectionSet::_inc_bytes_used_before

The G1CollectionSet::_inc_bytes_used_before variable is used to track allocations during mutator time, and at start of collection assigned to G1CollectionSet::_bytes_used_before, which then continues to track used bytes in the collection set, and that total result used for statistics.

There is no need to have the extra G1CollectionSet::_inc_bytes_used_before; there is no difference to just tracking incremental collection set used bytes directly in G1CollectionSet::_bytes_used_before, so remove it.