Remove unused tracker_mutex

[vil02]

This resolves #566.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.08%. Comparing base (4523047) to head (52b944e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
- Coverage   97.13%   97.08%   -0.05%     
==========================================
  Files          98       98              
  Lines       11290    11289       -1     
  Branches      796      796              
==========================================
- Hits        10966    10960       -6     
- Misses        324      329       +5     

Flag	Coverage Δ
unittests	97.08% <ø> (-0.05%)	⬇️

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