Retire ImmediateOfferKilled

[a1q123456]

High Level Overview of Change

Context of Change

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (non-breaking change that only restructures code)
• Performance (increase or change in throughput and/or latency)
• Tests (you added tests for code that already exists, or your new feature included in this PR)
• Documentation update
• Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• Release

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.3%. Comparing base (12c4b5a) to head (fd8ca10).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5973   +/-   ##
=======================================
  Coverage     78.3%   78.3%           
=======================================
  Files          817     817           
  Lines        69001   69001           
  Branches      8339    8338    -1     
=======================================
+ Hits         54021   54023    +2     
+ Misses       14980   14978    -2     

Files with missing lines	Coverage Δ
src/xrpld/app/tx/detail/CreateOffer.cpp	92.9% <100.0%> (ø)

... and 2 files with indirect coverage changes

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