Path fault should be removed if a node on faulted path is removed.

Once path fault is detected on vBridge, it retains even if a node on faulted path
is removed.

How to reproduce:

1. Run mininet and VTN Manager.
2. Configure one vBridge, and map untagged network by VLAN mapping.
3. Disable inter-switch link on mininet console.
4. Send a packet via disabled link. A path fault will be detected on the vBridge.
5. Stop mininet. A path fault retains in the vBridge though all switches are
disconnected.

VTN Manager should remove path faults that contain removed node.