undo CL 13348045 / 43675523c526

There is no reason to do this, and it's more work.

««« original CL description
net: make channel-based semaphore depend on receive, not send

R=r, dvyukov
CC=golang-dev
https://golang.org/cl/13348045

»»»

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13632047