fixes #153: Remove unused static instance of RESTServicePlugin

The instance is unused, but isn't cleaned up with the lifecycle of the plugin. This can prevent the plugin's class loader from being completely destroyed when the plugin is unloaded/reloaded.

src/java/org/jivesoftware/openfire/plugin/rest/RESTServicePlugin.java

@@ -34,9 +34,6 @@
  * The Class RESTServicePlugin.
  */
 public class RESTServicePlugin implements Plugin, PropertyEventListener {
-
-    /** The Constant INSTANCE. */
-    public static final RESTServicePlugin INSTANCE = new RESTServicePlugin();
 
     private static final String CUSTOM_AUTH_FILTER_PROPERTY_NAME = "plugin.restapi.customAuthFilter";
     public static final String SERVICE_LOGGING_ENABLED = "plugin.restapi.serviceLoggingEnabled";
@@ -67,16 +64,6 @@ public void setServiceLoggingEnabled(boolean serviceLoggingEnabled) {
     /** The custom authentication filter */
     private String customAuthFilterClassName;
 
-
-    /**
-     * Gets the single instance of RESTServicePlugin.
-     *
-     * @return single instance of RESTServicePlugin
-     */
-    public static RESTServicePlugin getInstance() {
-        return INSTANCE;
-    }
-
     /* (non-Javadoc)
      * @see org.jivesoftware.openfire.container.Plugin#initializePlugin(org.jivesoftware.openfire.container.PluginManager, java.io.File)
      */