[release-branch.go1.7] doc: remove mention of Go 1.6.3 working on Sierra

We thought it would at the time, but then Beta 4 changed the ABI
again, so it wasn't true in practice.

Fixes #17643

Change-Id: I36b747bd69a56adc7291fa30d6bffdf67ab8741b
Reviewed-on: https://go-review.googlesource.com/32238Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/32270

doc/go1.7.html

@@ -67,8 +67,7 @@ This change has no effect on the correctness of existing programs.
 
 <p>
 Go 1.7 adds support for macOS 10.12 Sierra.
-This support was backported to Go 1.6.3.
-Binaries built with versions of Go before 1.6.3 will not work
+Binaries built with versions of Go before 1.7 will not work
 correctly on Sierra.
 </p>