Remove unused get_state_msg method (backport #1949)

[mergify]

The get_msg will be removed upstream
ros-controls/realtime_tools#422

---

This is an automatic backport of pull request #1949 done by Mergify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.66%. Comparing base (f447716) to head (dc83fec).
⚠️ Report is 3 commits behind head on jazzy.

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #1950   +/-   ##
=======================================
  Coverage   84.66%   84.66%           
=======================================
  Files         138      138           
  Lines       13408    13408           
  Branches     1197     1197           
=======================================
  Hits        11352    11352           
  Misses       1643     1643           
  Partials      413      413           

Flag	Coverage Δ
unittests	84.66% <ø> (ø)

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Files with missing lines	Coverage Δ
...ntroller/test/test_trajectory_controller_utils.hpp	84.19% <ø> (ø)

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