Retire Flow and FlowSortStrands

[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 78.6%. Comparing base (b550dc0) to head (3157ef1).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6054   +/-   ##
=======================================
  Coverage     78.6%   78.6%           
=======================================
  Files          818     818           
  Lines        68953   68938   -15     
  Branches      8244    8237    -7     
=======================================
- Hits         54175   54166    -9     
+ Misses       14778   14772    -6     

Files with missing lines	Coverage Δ
src/xrpld/app/paths/RippleCalc.cpp	80.6% <ø> (+5.6%)	⬆️
src/xrpld/app/paths/detail/StrandFlow.h	92.0% <100.0%> (+2.3%)	⬆️

... and 3 files with indirect coverage changes

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