fix delete topic incompletion

[jeanlyn]

We don't need to delete the path under /brokers/topics when we deleting the topic because kafka will help us to do this, otherwise the delete process will hung, and the next time we can not create the same topic.

[jeanlyn]

The test failure information:

 - delete topic *** FAILED ***
  Vector("cm-test", "cm-unit-test") contained "cm-unit-test" Failed to delete topic (TestClusterManagerActor.scala:243)

The root cause is the path under /broker/topics will not delete without the real kafka cluster. Hence, can we remove this check?