It's not clear we want to enshrine an io interface in which Size cannot
return an error. Because this requires more thought before committing
to the API, remove from Go 1.7.

Fixes #15818.

Change-Id: Ic4138ffb0e033030145a12d33f78078350a8381f
Reviewed-on: https://go-review.googlesource.com/23392Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>