Remove Neutron Midonet plugin file completely

override-defaults mechanism allows us to define
s_neutron_plugin_security_group before loading lib/neutron,
and we no longer need to have a plugin-specific file in
the master DevStack repository.

Change-Id: I37d5012e89cb3650e4f325b6d77d70f28f87d3e7
Depends-On: I5e02acd288e53dd06a369d348ec77ead57d476fd

Author: Akihiro Motoki