broke windows build

««« original CL description
runtime: ensure forward progress of runtime.Gosched() for locked goroutines
The removed code leads to the situation when M executes the same locked G again and again.
Fixes #4820.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7310096
»»»

TBR=dvyukov
CC=golang-dev
https://golang.org/cl/7343050