Remove dragonflow reference

Dragonflow was retired in 2018 and is now being retired. This removes a
documentation reference to it.

Change-Id: I24ab79482306a7c816b5242a981f1b508ff8f6ec
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>
diff --git a/doc/source/plugin-registry.rst b/doc/source/plugin-registry.rst
index 3ab4db4..eda5773 100644
--- a/doc/source/plugin-registry.rst
+++ b/doc/source/plugin-registry.rst
@@ -41,7 +41,6 @@
 openstack/devstack-plugin-open-cas       `https://opendev.org/openstack/devstack-plugin-open-cas <https://opendev.org/openstack/devstack-plugin-open-cas>`__
 openstack/devstack-plugin-pika           `https://opendev.org/openstack/devstack-plugin-pika <https://opendev.org/openstack/devstack-plugin-pika>`__
 openstack/devstack-plugin-zmq            `https://opendev.org/openstack/devstack-plugin-zmq <https://opendev.org/openstack/devstack-plugin-zmq>`__
-openstack/dragonflow                     `https://opendev.org/openstack/dragonflow <https://opendev.org/openstack/dragonflow>`__
 openstack/ec2-api                        `https://opendev.org/openstack/ec2-api <https://opendev.org/openstack/ec2-api>`__
 openstack/freezer                        `https://opendev.org/openstack/freezer <https://opendev.org/openstack/freezer>`__
 openstack/freezer-api                    `https://opendev.org/openstack/freezer-api <https://opendev.org/openstack/freezer-api>`__