Cooja: Allow running the simulation without motes.

Normally it is not possible to start a simulation without motes. While
 this nomally makes sense, it might be, that motes are added at a later
 point in time by plugins.
 Removing this requirement is not problem, as it is still not possible
 to start the simulation without registered events. Thus a plugin can now
 register an event in the future to add motes at that point of time.

tools/cooja/java/se/sics/cooja/Simulation.java

@@ -1104,9 +1104,6 @@ public class Simulation extends Observable implements Runnable {
    * @return True if simulation is runnable
    */
   public boolean isRunnable() {
-    if (motes.isEmpty()) {
-      return false;
-    }
     return isRunning || hasPollRequests || eventQueue.peekFirst() != null;
   }