cmd/compile, go/parser: disallow "type T = p.T" - must use "=>"

cmd/compile, go/parser: disallow "type T = p.T" - must use "=>"

I had added this originally so we can play with different notations
but it doesn't make sense to keep it around since gofmt will convert
a type alias declaration using "=" into one using "=>" anyhow. More
importantly, the spec doesn't permit it.

Change-Id: Icb010b5a9976aebf877e48b3ce9d7245559ca494
Reviewed-on: https://go-review.googlesource.com/32105
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>