Revert "cmd/api: treat a hex-y VERSION as devel and permit API changes"

This reverts commit 11d1c05f.
See #9296 for details.

Change-Id: I89a36351cb007836662f28a611af5616818b95fe
Reviewed-on: https://go-review.googlesource.com/1536


Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>