Now that ReadMemStats is fast (CL 34937), CL 36791 is not so
necessary, and causes confusion. See #20863

This was already partially reverted in CL 46612 but missed two of the
spots.

Fixes #20863

Change-Id: I1307a0f7b1f9e86e8b6ceaa6a677f24f13431110
Reviewed-on: https://go-review.googlesource.com/47350Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>