NFSv4: Remove bogus call to nfs4_drop_state_owner() in _nfs4_open_expired()

There should be no need to invalidate a perfectly good state owner just
because of a stale filehandle. Doing so can cause the state recovery code
to break, since nfs4_get_renew_cred() and nfs4_get_setclientid_cred() rely
on finding active state owners.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>