Remove on_worker_started from Plugin base ABC class

[PaulM5406]

Hey again,

Remove Plugin.on_worker_started as it is not called anywhere and I think this is replaced with events.

Thanks !

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.31%. Comparing base (4026904) to head (ae001ab).
⚠️ Report is 2 commits behind head on 24_4.

Additional details and impacted files

@@            Coverage Diff             @@
##             24_4      #47      +/-   ##
==========================================
+ Coverage   71.26%   71.31%   +0.04%     
==========================================
  Files          55       55              
  Lines        2899     2893       -6     
==========================================
- Hits         2066     2063       -3     
+ Misses        833      830       -3     

Flag	Coverage Δ
unittests	71.31% <ø> (+0.04%)	⬆️

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