Remove CMAKE_CXX_STANDARD (backport #1704)

[mergify]

because it is defined here now: ros-controls/ros2_control_cmake#7

---

This is an automatic backport of pull request #1704 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.86%. Comparing base (6efe723) to head (01fb41f).
Report is 3 commits behind head on jazzy.

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #1706   +/-   ##
=======================================
  Coverage   84.86%   84.86%           
=======================================
  Files         127      127           
  Lines       12146    12146           
  Branches     1036     1036           
=======================================
  Hits        10308    10308           
  Misses       1499     1499           
  Partials      339      339           

Flag	Coverage Δ
unittests	84.86% <ø> (ø)

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