refactor: remove unused credential signature hash prefix

[mvadari]

High Level Overview of Change

This PR removes HashPrefix::credential, which is not used anywhere.

Context of Change

Drive-by fix

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

N/A

Test Plan

CI passes.

[Copilot]

Pull request overview

This PR removes the unused HashPrefix::credential enum value from the HashPrefix enumeration. This is a simple cleanup that removes dead code with no functional impact.

• Removes the HashPrefix::credential enum value that was defined but never used in the codebase

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.1%. Comparing base (07ff532) to head (0df6846).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6186   +/-   ##
=======================================
  Coverage     79.1%   79.1%           
=======================================
  Files          836     836           
  Lines        71260   71260           
  Branches      8306    8299    -7     
=======================================
+ Hits         56392   56396    +4     
+ Misses       14868   14864    -4     

Files with missing lines	Coverage Δ
include/xrpl/protocol/HashPrefix.h	100.0% <ø> (ø)

... and 4 files with indirect coverage changes

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