Currently, findRunnableGCWorker will perform mark completion if there
is no remaining work and no running workers. This used to be necessary
to resolve a race in the transition from mark 1 to mark 2 where we
would enter mark 2 with no mark work (and no dedicated workers), so no
workers would run, so no worker would signal mark completion.

However, we're about to make mark completion also perform the entire
follow-on process, which includes mark termination. We really don't
want to do that in the scheduler if it happens to detect completion.

Conveniently, this hack is no longer necessary because we always
enqueue root scanning work at the beginning of both mark 1 and mark 2,
so a mark worker will always run. Hence, we can simply eliminate it.

Change-Id: I3fc8f27c8da632f0fb732c9f6425e1f457f5652e
Reviewed-on: https://go-review.googlesource.com/16358Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>