refactor: LEAP-1308: Remove Stale Feature Flag - ff_back_dev_3865_filters_anno_171222_short

[luarmr]

PR fulfills these requirements

• Commit message(s) and PR title follows the format [fix|feat|ci|chore|doc]: TICKET-ID: Short description of change made ex. fix: DEV-XXXX: Removed inconsistent code usage causing intermittent errors
• Tests for the changes have been added/updated (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)
• Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• Self-reviewed and ran all changes on a local instance (for bug fixes/features)

Change has impacts in these area(s)

(check all that apply)

• Product design
• Backend (Database)
• Backend (API)
• Frontend

Describe the reason for change

Removing a FF that was stale for a few months.
Changes in LSE

WHEN REVIEW
CHECK THE STALE VALUE FIRST

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✅ Deploy Preview for label-studio-docs-new-theme canceled.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.70%. Comparing base (f293118) to head (89628a2).
Report is 15 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6744   +/-   ##
========================================
  Coverage    76.70%   76.70%           
========================================
  Files          170      170           
  Lines        13919    13919           
========================================
  Hits         10677    10677           
  Misses        3242     3242           

Flag	Coverage Δ
pytests	76.70% <ø> (ø)

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