mds: Remove some unused #defines

Signed-off-by: John Spray <john.spray@inktank.com>
mds/CDir: Remove a few lines of cruft

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Don't create the /.ceph directory

This was entirely unused.  On existing filesystems
it will live on, on new ones it will never be created.

Signed-off-by: John Spray <john.spray@inktank.com>
mds: Table read failure should log at error level

Signed-off-by: John Spray <john.spray@inktank.com>