cmd/link: minor code cleanup in dwarf gen

Minor code cleanup to get rid of a few unused parameters and return
values in the linker's dwarf generation code. No functional changes.

Change-Id: I1a68ebe0f08d8d32ca7adfdd2fb9db573a4fd5f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/332070


Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>