Remove server-side validation added in Y24-052, and corresponding feature flag

[KatyTaylor]

We attempted to get people to fix the production data but it was not prioritised. Removing this now as it just adds technical debt.

Closes #

Changes proposed in this pull request

• remove feature flag and undo server-side validation - because the invalid production data that blocks the release of this feature will likely never be fixed

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[StephenHulme]

Great, thank you!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.17%. Comparing base (17d8fb9) to head (e312f79).
⚠️ Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5411      +/-   ##
===========================================
+ Coverage    87.11%   87.17%   +0.06%     
===========================================
  Files         1457     1457              
  Lines        32589    32578      -11     
  Branches      3400     3400              
===========================================
+ Hits         28389    28399      +10     
+ Misses        4186     4165      -21     
  Partials        14       14              

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