osdc/Objecter: drop bad session nref assert

osdc/Objecter: drop bad session nref assert

This is a bad assert.  Specifically, handle_osd_op_reply may still be
holding the session ref while it is calling the completion for a previous
request.  This is safe: it is only holding the session ref after it dropped
the global map rwlock because of the per-session completion locks.  The
request in question was already marked completed by the time our thread
took the session lock.

Fixes: #9241
Signed-off-by: Sage Weil <sage@redhat.com>