Response to code review feedback on CL 40693.

Remove the final reference to it from package gc,
and manually unexport.

Passes toolstash-check.

Change-Id: I7fc48edd43263d8f7c56b47aeb7573408463dc22
Reviewed-on: https://go-review.googlesource.com/41303
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>