libceph: kill most of ceph_msg_pos

All but one of the fields in the ceph_msg_pos structure are now
never used (only assigned), so get rid of them.  This allows
several small blocks of code to go away.

This is cleanup of old code related to:
    http://tracker.ceph.com/issues/4428



Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>