virtio: drop unused virtio_device_stop_ioeventfd() function

virtio_device_stop_ioeventfd() has not been used since commit
310837de ("virtio: introduce
grab/release_ioeventfd to fix vhost") in 2016.

Nowadays ioeventfd is stopped implicitly by the virtio transport when
lifecycle events such as the VM pausing or device unplug occur.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191021150343.30742-1-stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>