virtiofsd: Remove useless code about send_notify_iov

The 'ch' will be NULL in the following stack:
send_notify_iov()->fuse_send_msg()->virtio_send_msg(), and
this may lead to NULL pointer dereferenced in virtio_send_msg().
But send_notify_iov() was never called, so remove the useless code
about send_notify_iov() to fix this problem.

Signed-off-by: Alex Chen <alex.chen@huawei.com>
Message-Id: <20201214121615.29967-1-alex.chen@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>