objecter: remove dead race handling code

objecter: remove dead race handling code

The only caller of _get_osd_session() holds rwlock for write, so
_get_session() cannot race. The only caller of _get_session() which
may possibly race is _op_submit(), which handles the race more simply
inline.

Call _get_session() inline in place of _get_osd_session(), and remove
the unused functions _get_osd_session() and
_promote_lock_check_race().

Signed-off-by: Josh Durgin <jdurgin@redhat.com>