Remove empty on_shutdown() callbacks

[Juliaj]

Address #1474

[saikishor]

@Juliaj can you also remove all other empty callbacks? Not only that of on_shutdown?. I believe we need this general cleanup

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.86%. Comparing base (7ddaf83) to head (8961e00).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1477      +/-   ##
==========================================
+ Coverage   83.83%   83.86%   +0.03%     
==========================================
  Files         122      122              
  Lines       11120    11113       -7     
  Branches      944      942       -2     
==========================================
- Hits         9322     9320       -2     
+ Misses       1489     1485       -4     
+ Partials      309      308       -1     

Flag	Coverage Δ
unittests	83.86% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...de/diff_drive_controller/diff_drive_controller.hpp	100.00% <ø> (ø)
...iff_drive_controller/src/diff_drive_controller.cpp	71.18% <ø> (+0.49%)	⬆️
...jectory_controller/joint_trajectory_controller.hpp	100.00% <ø> (ø)
...ory_controller/src/joint_trajectory_controller.cpp	83.93% <ø> (-0.03%)	⬇️
...nclude/tricycle_controller/tricycle_controller.hpp	100.00% <ø> (ø)
tricycle_controller/src/tricycle_controller.cpp	66.80% <ø> (+0.54%)	⬆️

... and 2 files with indirect coverage changes

[christophfroehlich]

Thanks for taking this one! We can remove the other callbacks also in a different PR if you like.

[saikishor]

If @christophfroehlich is fine with it. I'm fine with it ;)