Removed duplicate function declaration. Error in Google Closure Compiler

[albertdahlin]

This fix is commited on top of the 5.1.1 tag

Google Closure Compiler complains about this:

ERROR - Duplicate let / const / class / function declaration in the same scope is not allowed.
function badIndex(index, nestedProblems)
         ^^^^^^^^

The function badIndex is declared twice in the Native.Json module.
This looks like a PR merge mistake so I just removed the duplicate declaration.

[evancz]

This should be removed in the development branch of the library. Thanks for the report though!

[evancz]

This should be removed in the development branch of the library. Thanks for the report though!