Remove useless code in backend for network hooks

In backend NICs arrive with netinfo filled. If nic.network is not None
nic.netinfo is not too. Thus all the info is derived from HooksDict()
and nic.network must not be checked.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Helga Velroyen <helgav@google.com>