[dev.link] cmd/oldlink: remove tests

They are essentially duplicates of cmd/link tests. No need to
test twice.

Change-Id: I91fdc996f5e160631648ee63341c4e46bb6cc54d
Reviewed-on: https://go-review.googlesource.com/c/go/+/226801


Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>