Remove VERSION, which was forcing version to 'devel'.

Old:
$ go version
go version devel

New:
$ go version
go version devel +0a3866d6cc6b Mon Sep 24 20:08:05 2012 -0400

The date and time (and time zone) is that of the most recent commit,
not the time of the build itself. With some effort we could normalize
the zone, but I don't think it's worth the effort (more C coding,
since Mercurial is unhelpful).

R=r, dsymonds
CC=golang-dev
https://golang.org/cl/6569049