Consider changing default option for controller

[cmower]

When launching the bringup, the default controller is position_trajectory_controller, i.e.

https://github.com/KCL-BMEIS/lbr_fri_ros2_stack/blob/c1aadf3854830c8e54966238c17ca0047ef23501/lbr_bringup/launch/lbr_bringup.launch.py#L62-L66

I suggest the default controller should be none, and it should be left as a user option.

Also, the documentation should specify somewhere that these controllers refer to those from ROS control. This would make it clearer how to interface with the robot (i.e. what messages to send or actions to use).