With the hybrid barrier, rescanning stacks is no longer necessary so
the rescan list is no longer necessary. Remove it.

This leaves the gcrescanstacks GODEBUG variable, since it's useful for
debugging, but changes it to simply walk all of the Gs to rescan
stacks rather than using the rescan list.

We could also remove g.gcscanvalid, which is effectively a distributed
rescan list. However, it's still useful for gcrescanstacks mode and it
adds little complexity, so we'll leave it in.

Fixes #17099.
Updates #17503.

Change-Id: I776d43f0729567335ef1bfd145b75c74de2cc7a9
Reviewed-on: https://go-review.googlesource.com/36619
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>