SPIGOT-210: Remove the exception when dropping a non-existant task

If the server crashes during chunk generation then the chunk would have never
been added to the executor, this caused a second exception to be thrown when
the server shutdown from the first exception

src/main/java/org/bukkit/craftbukkit/util/AsynchronousExecutor.java

@@ -263,7 +263,7 @@ public final class AsynchronousExecutor<P, T, C, E extends Throwable> {
     public boolean drop(P parameter, C callback) throws IllegalStateException {
         final Task task = tasks.get(parameter);
         if (task == null) {
-            throw new IllegalStateException("Unknown " + parameter);
+            return true;
         }
         if (!task.callbacks.remove(callback)) {
             throw new IllegalStateException("Unknown " + callback + " for " + parameter);