Remove "plugins" directory
Remove "plugins" directory

It seems like a leftover from cisco driver removal. [1]

[1] I0767852e2b5643c1aa45c8d58a430c3e3f5d6d36

Change-Id: I4488b90cf728a4401a3c546572fd34ef24e571ec

openstack/neutron-fwaas,openstack/neutron-fwaas