Added login listener to prevent players from joining with an unsupported version

src/main/java/net/krakatoa/proxy/ProxySystem.java

@@ -97,10 +97,4 @@ public class ProxySystem extends Plugin {
           pluginManager.registerCommand(this, listener);
         });
   }
-
-  /*
-  TODO: proxy ping & join only accessible for 1.17.1 clients
-
-   */
-
 }