Remove FIXME and TODO in code base

eg: 
```
      case "BalancingDispatcher" =>
        // FIXME remove this case in Akka 2.4
        throw new IllegalArgumentException(
          "BalancingDispatcher is deprecated, use a BalancingPool instead. " +
          "During a migration period you can still use BalancingDispatcher by specifying the full class name: " +
          classOf[BalancingDispatcherConfigurator].getName)
```