Remove the RO Core

Now that the rw-core and ro-core APIs have been unified, there is no need to keep the ro-core.  It should be removed from the voltha-go repo.

If the ro-core needs to be referenced or recovered for any reason, it will remain in the git history.

Per discussion: The ro-core should be removed immediately following a release.  (Either a proper point release, or simply a git tag to reference the last available ro-core.)