We used to compute an estimate of the reachable heap size that was
different from the marked heap size. This ultimately caused more
problems than it solved, so we pulled it out, but memstats still has
both heap_reachable and heap_marked, and there are some leftover TODOs
about the problems with this estimate.

Clean this up by eliminating heap_reachable in favor of heap_marked
and deleting the stale TODOs.

Change-Id: I713bc20a7c90683d2b43ff63c0b21a440269cc4d
Reviewed-on: https://go-review.googlesource.com/29271
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>