cmd/link: remove dummy argument from ld.Errorf

cmd/link: remove dummy argument from ld.Errorf

As the comment notes, all calls to Errorf now pass nil,
so remove that argument entirely.

There is a TODO to remove uses of Errorf entirely, but
that seems wrong: sometimes there is no symbol on
which to report the error, and in that situation, Errorf is
appropriate. So clarify that in the docs.

Change-Id: I92b3b6e8e3f61ba8356ace8cd09573d0b55d7869
Reviewed-on: https://go-review.googlesource.com/c/go/+/625617
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>