skopeo: drop support for ostree

drop support for the ostree backend.  The only known user for the
backend is the atomic CLI tool that is not maintained anymore
upstream.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1766404

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

completions/bash/skopeo

@@ -44,7 +44,6 @@ _skopeo_copy() {
     --src-tls-verify
     --dest-creds --dcreds
     --dest-cert-dir
-    --dest-ostree-tmp-dir
     --dest-tls-verify
     --src-daemon-host
     --dest-daemon-host