rm ceph-extras repository config

rm ceph-extras repository config

This repository contains packages that we don't have time to continue
maintaining. In particular, the CentOS 6 QEMU packages were vulnerable
to VENOM (CVE-2015-3456) and no users should rely on them. The other
packages in "ceph-extras" will continue to incur security issues over
time.

With the recent migration from "ceph.com" -> "download.ceph.com", we've
chosen not to copy these "ceph-extras" files over to the new locations.