Remove HEDLEY annotation from exception::what()

[remyjette]

The latest MSVC compiler raises the following warning on
nlohmann::detail::exception::what() if /analyze is enabled:

vcruntime_exception.h(93) : warning C28204: 'what' has an override at
nlohmann\json\develop\single_include\nlohmann\json.hpp(2644)
and only the override is annotated for return: when an override is
annotated, the base (this function) should be similarly annotated.

See https://godbolt.org/z/r331h4

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
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[coveralls]

Coverage remained the same at 100.0% when pulling 0a9ec38 on remyjette:remove-exception-annotation into 176d8e2 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!