Fixes #206 (#210)

pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

             pluginsRoot = createPluginsRoot();
         }
 
-        System.setProperty("pf4j.pluginsDir", pluginsRoot.toString());
-
         pluginRepository = createPluginRepository();
         pluginFactory = createPluginFactory();
         extensionFactory = createExtensionFactory();