osd,osdc: eliminate FLAG_ONDISK and helpers

osd,osdc: eliminate FLAG_ONDISK and helpers

The objecter actually always needs to get a response in order to
be able to not continually resend ops (even if the caller didn't
provide a callback).  Thus, it makes no sense for an MOSDOp to
ever not have FLAG_ONDISK set.  Therefore, we'll just remove the
helper and assume it's always there (it's safe to send a response
the client didn't ask for, the error paths already do that).  On
the Objecter side, we'll just unconditionally fill in ONDISK for
the benefit of pre-luminous OSDs.

Fixes: http://tracker.ceph.com/issues/18961
Signed-off-by: Samuel Just <sjust@redhat.com>