The gcstats structure is no longer consumed by anything and no longer
tracks statistics that are particularly relevant to the concurrent
garbage collector. Remove it. (Having statistics is probably a good
idea, but these aren't the stats we need these days and we don't have
a way to get them out of the runtime.)

In preparation for #13613.

Change-Id: Ib63e2f9067850668f9dcbfd4ed89aab4a6622c3f
Reviewed-on: https://go-review.googlesource.com/34936
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>