Flow entries are not removed even if a path policy is modified.

If a path policy is created, modified, or removed by a path policy APIs,
the VTN Manager should remove all flow entries affected by the target path policy.
But the current VTN Manager does not change any flow entries.

VTN flow is designed to be associated with a path policy that determines
the physical route of packets. But the current VTN Manager never associate
VTN flow with path policy. So path policy APIs cannot select flow entries
affected by the target path policy.