Remove HEDLEY annotation from exception::what()

The latest MSVC compiler throws 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

include/nlohmann/detail/exceptions.hpp

@@ -49,7 +49,6 @@ class exception : public std::exception
 {
   public:
     /// returns the explanatory string
-    JSON_HEDLEY_RETURNS_NON_NULL
     const char* what() const noexcept override
     {
         return m.what();

single_include/nlohmann/json.hpp

@@ -2640,7 +2640,6 @@ class exception : public std::exception
 {
   public:
     /// returns the explanatory string
-    JSON_HEDLEY_RETURNS_NON_NULL
     const char* what() const noexcept override
     {
         return m.what();