Do not assert router l3 agent association

Just be removing the router from an agent, we does not prevent
neutron to reschedule the router to the same agent.

Since on dvsm gate we has only one l3 agent, it is the only scheduling
choose.

Closes-Bug: #1291694
Change-Id: Idf7d184d2493fef7f6faa17f0d95c1c9f853dfb1
Merge "Do not assert router l3 agent association"