fix: Remove duplicate mark_task_as_started listener

[tpoliaw]

Only one of the listeners was unsubscribed so they would pile up in the
WorkerEvent publisher.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.25%. Comparing base (79cae0b) to head (0fa876d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1435      +/-   ##
==========================================
- Coverage   95.25%   95.25%   -0.01%     
==========================================
  Files          43       43              
  Lines        3164     3163       -1     
==========================================
- Hits         3014     3013       -1     
  Misses        150      150              

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