Remove deprecated feedforward service from example 16

[astral-fi]

Removed set_feedforward_control from demo_test_helper.py
Removed Feedforward Service from the userdoc

[christophfroehlich]

Thanks for picking this!

Some comments in the code below. Do we need the demo_test_helper.py now? There is only a single twist command publisher left, I suggest removing this and updating the section (point 3 in Visualize the convergence of DiffBot's wheel velocities and commands), what do you think? or @Juliaj?

[astral-fi]

@christophfroehlich Hey! Thank you for helping me out. I have made changes as per your suggestion. Please let me know if I messed up somewhere else.

[Juliaj]

Do we need the demo_test_helper.py now? There is only a single twist command publisher left, I suggest removing this and updating the section (point 3 in Visualize the convergence of DiffBot's wheel velocities and commands), what do you think? or @Juliaj?

Let's remove demo_test_helper.py as its remaining logic is minimal, and the file is no longer needed.