chore: Remove the forward declarations that cause build errors when unity build is enabled

[a1q123456]

High Level Overview of Change

This PR removes the forward declarations from src/libxrpl/ledger/helpers/MPTokenHelpers.cpp because they cause build errors when unity build is on.

Context of Change

The forward declarations are not necessary and they cause build errors when unity build is on.

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 81.5%. Comparing base (0eedefb) to head (3ff5672).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #6633   +/-   ##
=======================================
  Coverage     81.4%   81.5%           
=======================================
  Files          997     997           
  Lines        74410   74410           
  Branches      7562    7554    -8     
=======================================
+ Hits         60598   60612   +14     
+ Misses       13812   13798   -14     

Files with missing lines	Coverage Δ
src/libxrpl/ledger/helpers/MPTokenHelpers.cpp	96.2% <100.0%> (ø)

... and 5 files with indirect coverage changes

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