Revert "runtime: implement runqdrain() for GC mark worker goroutines"

This reverts CL 310149.

Reason for revert: Breaks longtest builders:

https://build.golang.org/log/6af9fb147fa3101154db10e7ce055e8267cd4c93
https://build.golang.org/log/172ed6e1ec3bb503370333ee421c590fd2a72d0a

Change-Id: Iaf5a8b9eec51d0517311e050d0b0f7569759d292
Reviewed-on: https://go-review.googlesource.com/c/go/+/312129


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>