[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)
• [x] 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 :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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