Delete nav2_controller_example

[crdelsey]

Now that the DWB controller is working, we can delete nav2_controller_example.

When this is done, the reference to the controller should also be deleted from nav2_bringup/launch/core.launch.py

This example controller will eventually be replaced by a functional carrot controller as mentioned in #202

[SteveMacenski]

I'm fairly certain this was done already in a recently merged PR

[crdelsey]

You're right! I think that was the next commit after I submitted this :-)