Stop configuring neutron to enforce DHCP MTU option

Since Mitaka, Neutron does the right job to calculate and advertise
correct MTU to instances, out of the box [1].

[1] https://review.openstack.org/#/c/302165/

TrivialFix
Change-Id: I71a6dc5b56c2c1fb9db757df77a56feababdc039