Revert "vdsm: bridge: handle engine at loopback (all-in-one)"

ovirt-engine cannot cope with empty bridge (bridge without interfaces
assigned).

Network team does not wish to fix engine.

We will have to drag workarounds for it.

This reverts commit 817103dfd95f00d894e54730b3789aa8a9cf1e63.

Change-Id: If4a575a6e48a33ca346b911b99243177de6c46a9
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>