Remove custom PID controller from DXL envs

[csherstan]

Both DxlReacher1DEnv and DxlTracker1DEnv use a custom PID controller to return the motor to the starting position. This is unnecessary - it is straightforward to switch to position control.

However, to implement this change would require reworking the DXLCommunicator and providing a mechanism for specifying different control modes as the communicator assumes torque control.