runtime: eliminate gchelper mechanism

Now that we do no mark work during mark termination, we no longer need
the gchelper mechanism.

Updates #26903.
Updates #17503.

Change-Id: Ie94e5c0f918cfa047e88cae1028fece106955c1b
Reviewed-on: https://go-review.googlesource.com/c/134785


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>