Clean up routes on FIP disassociation.

* The update of the floatingip_db was happening before the disassociation, so
  the disassociation happened on new data.
* The old data was required to identify the router_id.
* This fix changes the order of the disassociation and the floatingip_db
  update.

Fixes MN-985
Signed-off-by: Joe Mills <joe@midokura.com>