MAINTAINERS: remove myself as ceph co-maintainer

Jeff, Ilya, and Dongsheng are doing all of the Ceph maintainance
these days.

[ idryomov: Remove Sage's git tree too, it hasn't been pushed to
  in years. ]

Signed-off-by: Sage Weil <sage@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>