And start deleting from go1.8.txt.

TBR=See https://golang.org/cl/33244

Updates #17929

Change-Id: I71011d97b23a7ba94cd51e16ae61fda18e8b96eb
Reviewed-on: https://go-review.googlesource.com/33680Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>