Remove min_l3_agents_per_router option

min_l3_agents_per_router parameter has been removed in neutron[0].
So we can remove it.

[0] https://github.com/openstack/neutron/blob/master/releasenotes/notes/remove-min-l3-agents-per-router-27aef7d91dec0348.yaml

Change-Id: Ia3f3686e2cc27e4bd0707add50ebc552117d6b03
Closes-Bug: #1677101