network: drop back compat code loading actual bridge name

network: drop back compat code loading actual bridge name

The actual network def was updated to save the bridge name back
in 1.2.11:

  commit a3609121799d44898a3e0d0bf92b755e55e7b418
  Author: Laine Stump <laine@laine.org>
  Date:   Fri Nov 21 12:20:37 2014 -0500

    network: save bridge name in ActualNetDef when actualType==network too

The chance that someone is running libvirt < 1.2.11 and wants
todo a live upgrade to 5.3.0 without a host reboot is essentially
zero. We can thus reasonably drop the back compat code now.

Reviewed-by: Laine Stump <laine@laine.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>