virtio-net: remove useless disable on freeze

disable_cb is just an optimization: it
can not guarantee that there are no callbacks.
In particular it doesn't have any effect when
event index is on.

Instead, detach, napi disable and reset on freeze ensure we don't run
concurrently with a callback.

Remove the useless calls so we get same behaviour
with and without event index.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>