VFP leak in windows overlay network deletion

[trapier]

Issue Summary

Windows hosts experience an accretion of VFP load balancers during network removal.

Steps to Recreate

1. Install jq on a windows host and join it to a swarm
2. Deploy a singly replicated windows service
3. Observe set of load balancers:

   hnsdiag list loadbalancers -d | jq '[.ID,.Policies[0].SourceVIP,.Policies[0].VIPs[0]]|@tsv' -r
   

4. Drain windows host or scale service to 0, then undrain or scale service to 1
5. Repeat steps 3 and 4, watching the number of load balancers grow.

Relevant Commit

c47239d#diff-cba6b1b3488fe48ebbef7e1821a3745dL1081 (#2138)