Retire fix1623 amendment

[pratikmankawde]

High Level Overview of Change

This change retires the fix1623 amendment.

Context of Change

Amendments activated for more than 2 years can be retired.
https://xrpl.org/resources/known-amendments#fix1623

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.3%. Comparing base (44e027e) to head (208559e).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5928   +/-   ##
=======================================
  Coverage     78.3%   78.3%           
=======================================
  Files          817     817           
  Lines        68944   68928   -16     
  Branches      8324    8301   -23     
=======================================
+ Hits         53965   53977   +12     
+ Misses       14979   14951   -28     

Files with missing lines	Coverage Δ
src/xrpld/app/tx/detail/CashCheck.cpp	92.0% <100.0%> (-<0.1%)	⬇️
src/xrpld/rpc/detail/DeliveredAmount.cpp	84.0% <100.0%> (-2.2%)	⬇️

... and 7 files with indirect coverage changes

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[a1q123456]

LGTM