Revert "bufio: make Reader.Peek invalidate Unreads"

Revert "bufio: make Reader.Peek invalidate Unreads"

Go 1.9 beta 2 is already out.
It's too late to break code with a change like this.
This can be rolled forward for Go 1.10.

This reverts commit ae238688d2813e83f16050408487ea34ba1c2fff.

Change-Id: Ib67b8629e3deac5d50d76581aba6a91ca7a7853e
Reviewed-on: https://go-review.googlesource.com/47570
Reviewed-by: Russ Cox <rsc@golang.org>