Clean conntrack records when removing floating ip

Established connections via floating ip won't get disconnected after
disassociating.
This patch move clean_conntrack from migration code to remove_floating_ip to
make sure that established connections on floating ip will get interrupted
immediately after disassociating.

Change-Id: I88d258ea39ea7c76e4cc60a636eb1e3e6aaa6f22
Closes-Bug: #1334938
Merge "Clean conntrack records when removing floating ip"