[TRIVIAL] Remove unused lambdas from MultiApiJson_test

[Bronek]

High Level Overview of Change

This fixes "unused lambda" compiler warning, which in some settings could break compilation.

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-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.96%. Comparing base (c187f75) to head (d992e63).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4980      +/-   ##
===========================================
- Coverage    76.96%   76.96%   -0.01%     
===========================================
  Files         1129     1129              
  Lines       131913   131914       +1     
  Branches     39579    39673      +94     
===========================================
- Hits        101530   101528       -2     
- Misses       24424    24491      +67     
+ Partials      5959     5895      -64     

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[scottschurr]

Looks great. Thanks!