Stale flows after clearing gateway info from a router

After clearing gateway info from a router there are still flows related to Floating IPs, the gateway and an external PNF.

How to reproduce

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1. Create an internal network, external network
2. Create a router and add internal interface and add the gateway information
3. Launch instances
4. Create floating IPs and associate them with the instances
5. Delete the instances and the Floating IPs
6. Remove the gateway info from the router and verify there are stale flows.

Version

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Oxygen opendaylight-8.0.0-2.el7ost.noarch

Additional info

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Added a log from a CSIT test