mds: do not register as a service daemon

mds: do not register as a service daemon

This commit undoes the service daemon registration for the MDS. It doesn't look
absolutely necessary and it causes the MDS to be listed twice in the `ceph
versions` output:

    $ ceph versions
        ...
        "mds": {
            "ceph version v15.0.0-6915-g0143b904676 (0143b9046763ea1801efa8358a0c033ec862cea9) octopus (dev)": 3
        },
        "mds": {
            "unknown": 3
        },
        "overall": {
            "ceph version v15.0.0-6915-g0143b904676 (0143b9046763ea1801efa8358a0c033ec862cea9) octopus (dev)": 10,
            "unknown": 3
        }

Fixing that requires looking for duplicates or ignoring MDSs in the
service daemons when the mon processes `ceph versions`. I have a feeling
that it wasn't actually designed to be used by the MDS this way however.
Additionally, the reason for "unknown" version is because the metadata
sent to the mgr does not include "ceph_version".

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit 046137f819aae72f1423e3feb213f0e46c97c9ce)