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

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

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.18%. Comparing base (dfbf1d9) to head (db20f1e).
Report is 1 commits behind head on jazzy.

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Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #3238      +/-   ##
==========================================
+ Coverage   44.18%   44.18%   +0.01%     
==========================================
  Files         698      698              
  Lines       61588    61585       -3     
  Branches     7459     7459              
==========================================
  Hits        27207    27207              
+ Misses      34213    34210       -3     
  Partials      168      168              

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