move TrajectoryExecutionManager::clear() to private (backport #3226)

[mergify]

Description

previously discussed here #3198 (comment)

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

---

This is an automatic backport of pull request #3226 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 3ec75ab has failed:

On branch mergify/bp/humble/pr-3226
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Unmerged paths:
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	deleted by us:   moveit_py/src/moveit/moveit_ros/trajectory_execution_manager/trajectory_execution_manager.cpp
	both modified:   moveit_ros/move_group/src/default_capabilities/execute_trajectory_action_capability.cpp
	both modified:   moveit_ros/planning/plan_execution/src/plan_execution.cpp
	deleted by us:   moveit_ros/planning/trajectory_execution_manager/include/moveit/trajectory_execution_manager/trajectory_execution_manager.hpp

no changes added to commit (use "git add" and/or "git commit -a")

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[sea-bass]

@rhaschke Technically this changes the API for Humble, which is not ideal... but it's pretty unlikely that people are using this clear() method. However, interested in your thoughts on whether this + the other PR (#3239) should actually make it back to Humble.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.41%. Comparing base (7a425d9) to head (4ae0a31).
Report is 2 commits behind head on humble.

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Additional details and impacted files

@@           Coverage Diff           @@
##           humble    #3237   +/-   ##
=======================================
  Coverage   51.41%   51.41%           
=======================================
  Files         382      382           
  Lines       31893    31893           
=======================================
  Hits        16396    16396           
  Misses      15497    15497           

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[rhaschke]

Yes, I think so. I backported it to MoveIt1 as well.