external network flows remain after external network deleted

ODL version: 6.2
distribution-karaf-0.6.2-20170721.171913-136.tar.gz

Flow entries in table 52 and 55 for external network remain after the network is deleted.

Reproduction steps:

1) Create external network

neutron net-create ext-net --router:external --provider:physical_network public --provider:network_type flat

2) The following flows for the external ELAN installed:

cookie=0x8701389, duration=12.429s, table=52, n_packets=0, n_bytes=0, priority=5,metadata=0x1389000001/0xffff000001 actions=write_actions(group:210001)
cookie=0x8701389, duration=12.427s, table=52, n_packets=0, n_bytes=0, priority=5,metadata=0x1389000000/0xffff000001 actions=write_actions(group:210002)
cookie=0x8800001, duration=12.429s, table=55, n_packets=0, n_bytes=0, priority=10,tun_id=0x1,metadata=0x10000000000/0xfffff0000000000 actions=drop
cookie=0x8800001, duration=12.429s, table=55, n_packets=0, n_bytes=0, priority=9,tun_id=0x1 actions=load:0x100->NXM_NX_REG6[],resubmit(,220)

3) Delete the external network

neutron net-delete ext-net

4) The flow entries in 2) remain. They should have been deleted when the associated external network deleted