remove unused plugin.get_plugin_name()
remove unused plugin.get_plugin_name()

This method does not seem to be used anymore:
http://codesearch.openstack.org/?q=get_plugin_name&i=nope&files=&repos=neutron,neutron-lib

Change-Id: I634307bd996eebeabb69f17587d8132e362ec900

openstack/tap-as-a-service,stackforge/tap-as-a-service,stackforge/tap-as-a-service,openstack/tap-as-a-service