registry: don't iterate through certs

the golang tls.Conn does a fine job of that.
http://golang.org/src/pkg/crypto/tls/handshake_client.go?#L334

More backstory: https://botbot.me/freenode/docker-dev/msg/23510546/

ping @tiborvass @dmp42 
Signed-off-by: Vincent Batts <vbatts@redhat.com>