net_scale: avoid netinfo instantiation for removal check

When removing networks, delNetwork used to generate a whole NetInfo
instance, which is rather costly and in the case that there's a lot
of delNetworks queued up (like in a lengthy setupNetworks operation)
it can impact performance significantly.

We should rely on:
 - configurator.removeLibvirtNetwork
 - configurator removal methods

Bug-Url: https://bugzilla.redhat.com/1053114
Change-Id: I1ff62dcd750ece1c3685c102723820aac5b432b2
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24158
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>