refactor: OPTIC-1621: Remove Stale Feature Flag - fflag_perf_back_lsd…

[luarmr]

Restore a reverted PR and check if the upstream automatically picks the LSE PR.

#7002

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[bmartel]

Reapproving based on the original

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.94%. Comparing base (06eeb5c) to head (b6054dc).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/projects/models.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7014      +/-   ##
===========================================
+ Coverage    76.91%   76.94%   +0.02%     
===========================================
  Files          175      175              
  Lines        14167    14155      -12     
===========================================
- Hits         10896    10891       -5     
+ Misses        3271     3264       -7     

Flag	Coverage Δ
pytests	76.94% <83.33%> (+0.02%)	⬆️

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