os/exec: delete RunContext, WaitContext

[rsc]

See discussion on https://go-review.googlesource.com/#/c/22529
We should do this before Go 1.7 since this is otherwise new API we are stuck with.

[bradfitz]

https://golang.org/cl/23300

[gopherbot]

CL https://golang.org/cl/23300 mentions this issue.