Remove model-topology

The opendaylight-topology*.yang models are the very first cut at
modeling a network. As such they have been superseded by standard-track
draft ietf-topology, which evolved into ietf-network.

There is noone using these old models in OpenDaylight proper, which
means they are just a maintenance burden and serve only to confuse
newcomers.

JIRA: CONTROLLER-1978
Change-Id: I051781d05f1caf10b9b3c65a4f178b20162adad6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>