Remove openvswitch core plugin entry point

The openvswitch core plugin has been removed but not its associated
"neutron.core_plugins" entry point. This change removes it from
setup.cfg.

Change-Id: I79f7c334cdeb0f4d0d68743734f69c0ec8523467
Related-Bug: #1323729
Closes-Bug: #1391326
(cherry picked from commit f36c29903afb713670f9e7c3f9530e117dfc6bcb)