When deadline has already passed,
current context is canceled before return cancel function.
So is unnecessary to call cancel with remove from parent again
in return cancel function.

Change-Id: I37c687c57a29d9f139c7fb648ce7de69093ed623
Reviewed-on: https://go-review.googlesource.com/c/50410
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>