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>