MB-8497: Remove unnecessary memory accounting code

This extra code was needed because of the alignment stuff
in the previous commit. The new code actually gets the same
numbers but using less lines of code and less atomic ops.

Change-Id: Icb15a1d07d93058f3c3d0bdc2307ac6eee698342
Reviewed-on: http://review.couchbase.org/27036
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>