cmd/link: clean up some comments to Go standards

Also drop a couple of unused names.

Change-Id: I0f09775a276c34ece7809cf5d3f7c6b8cd1fa487
Reviewed-on: https://go-review.googlesource.com/c/go/+/718580
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ianlancetaylor