Remove the router_interface_to_key in db_store
Remove the router_interface_to_key in db_store

router_interface_to_key was added in [1], and obsoleted in [2][3].
It is not used in db_store now.

[1] https://review.openstack.org/#/c/208184/
[2] https://review.openstack.org/#/c/216165/
[3] https://review.openstack.org/#/c/234178

Change-Id: I03af02926c06bb62c1dcaff8d8207ad765e8e525

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow