cmd/link: remove darwin/arm test

The darwin/arm port is removed in Go 1.15. Setting GOOS=darwin
GOARCH=arm will fail, therefore "go test cmd/link" on macOS will
fail (in non -short mode). Remove this test point.

Updates #37611.

Change-Id: Ia9531c4b4a6692a0c49153517af9fdddd1f3e0bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/227341


Reviewed-by: Ian Lance Taylor <iant@golang.org>