Y25-709 - Remove obsolete accessioning code

[StephenHulme]

Closes #5420 and #4878

Changes proposed in this pull request

• Remove y25_286_accession_individual_samples_with_sample_accessioning_job feature flag
• Remove references to Accessionable::Sample
• Remove references to accession_local_key and contact_user (see deployment PR)
• Remove FakeAccessionService and references
• Remove sample-accessioning-related Cucumber tests
• Convert study-accessioning-related Cucumber tests to Rspec feature tests

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.23%. Comparing base (cae30e7) to head (90e3a01).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5484      +/-   ##
===========================================
- Coverage    87.35%   87.23%   -0.13%     
===========================================
  Files         1462     1459       -3     
  Lines        32966    32851     -115     
  Branches      3461     3449      -12     
===========================================
- Hits         28796    28656     -140     
- Misses        4149     4174      +25     
  Partials        21       21              

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[andrewsparkes]

Yay lots of deleting.

[yoldas]

Big cleanup.