undo CL 7310096 / 59da6744d66d

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 issue 4820.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7310096
???

TBR=dvyukov
CC=golang-dev
https://codereview.appspot.com/7343050