Revert "context: use Go 1.8 type alias for CancelFunc and Context"

This reverts commit dffc95cbb4ff9af8614b2741ec26371fffcc19b3.

Aliases aren't going into Go 1.8. See:
https://github.com/golang/go/issues/16339#issuecomment-258527920

Updates #16339

Change-Id: Ie37e37349db596a89b4944179ab87b6f0577c826
Reviewed-on: https://go-review.googlesource.com/32753
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>