Remove old plugin system

Remove the old client/instance fused plugin system along with
instanceManagement in client.js and getPlugins in master.js.

test/missing.js

@@ -2,7 +2,6 @@
 // required here in order to track coverage for them.
 
 require('lib/manager/modManager');
-require('lib/manager/pluginManager');
 require('lib/npmPostinstall.js');
 require('lib/typedef');
 require('../clusterctl.js');