The combination of the changes for 6494965, 6888336, and 7127706 the field _top_at_conc_count should no longer be needed.
With 6888336 and 7127706, the value of NTAMS (for each region) should not change as a result of performing GCs during the marking cycle.
As a result the value that we currently record in _top_at_conc_count should not change between remark and cleanup. Therefore the field can be removed.
At the same time, the code that adds [_top_at_conc_count, ntams) to the liveness counting (during cleanup) should now be a nop and can also be removed.