move TrajectoryExecutionManager::clear() to private

[patrickKXMD]

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

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.64%. Comparing base (a77eb98) to head (1b82272).
Report is 3 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3226      +/-   ##
==========================================
- Coverage   45.67%   45.64%   -0.02%     
==========================================
  Files         714      714              
  Lines       62299    62293       -6     
  Branches     7531     7529       -2     
==========================================
- Hits        28446    28425      -21     
- Misses      33687    33701      +14     
- Partials      166      167       +1     

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[rhaschke]

I generally approve.

[patrickKXMD]

Updated with suggestions applied, thanks!