remove old bundled mage-theme-plugin.jar

which causes plugin loading to consistently fail, so switch back to
directly instantiating ThemePluginImpl()

this is a little unfortunate, because these plugins don't get the same
lifecycle handling as external ones

re: c25e45095e

Mage.Client/src/main/java/org/mage/plugins/card/CardPluginImpl.java

@@ -81,6 +81,7 @@ public class CardPluginImpl implements CardPlugin {
 
     @PluginLoaded
     public void newPlugin(CardPlugin plugin) {
+        log.info(plugin.toString() + " has been loaded.");
     }
 
     @Override