arm/armspec: remove obsolete comment

I didn't realize it at the time, but this comment was referring
to the build constraint that was removed in CL 534197.

Updates golang/go#12840.

Change-Id: I1e6694a6c05f4b2e6dbffe6488c69f9d530bdaf4
Reviewed-on: https://go-review.googlesource.com/c/arch/+/534221
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

arm/armspec/spec.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// ... see golang.org/issue/12840
-
 // Armspec reads the “ARM Architecture Reference Manual”
 // to collect instruction encoding details and writes those details to standard output
 // in JSON format.