runtime: remove unused gcstats
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>