Retire fixSTAmountCanonicalize code

[pratikmankawde]

High Level Overview of Change

This change retires the fixSTAmountCanonicalize amendment.

Context of Change

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

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

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.3%. Comparing base (80a3ae6) to head (b510fea).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/libxrpl/protocol/STAmount.cpp	33.3%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5956     +/-   ##
=========================================
- Coverage     78.3%   78.3%   -0.0%     
=========================================
  Files          817     817             
  Lines        68963   68944     -19     
  Branches      8307    8303      -4     
=========================================
- Hits         54007   53990     -17     
+ Misses       14956   14954      -2     

Files with missing lines	Coverage Δ
include/xrpl/protocol/STAmount.h	95.3% <ø> (-0.2%)	⬇️
src/xrpld/app/misc/detail/TxQ.cpp	97.6% <ø> (-<0.1%)	⬇️
src/xrpld/app/tx/detail/Transactor.cpp	91.9% <ø> (-<0.1%)	⬇️
src/xrpld/app/tx/detail/apply.cpp	92.8% <ø> (-0.1%)	⬇️
src/libxrpl/protocol/STAmount.cpp	88.9% <33.3%> (+<0.1%)	⬆️

... and 4 files with indirect coverage changes

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