Unused macro, misleading comments regarding `sscanf`

[dan-so]

Commit 96150ba removed all calls to sscanf, but it's still referenced in a comment here:
https://github.com/kgabis/parson/blob/master/parson.c#L594

I don't know how much (if any) of this comment is still accurate.

This same commit added the line
#define sscanf THINK_TWICE_ABOUT_USING_SSCANF

This is a logical way to prevent use of this function, but recent compilers will warn about unused macros (and often this warning is enabled by -Wall or -Wextra).

I propose either of these fixes to remove this warning:

• explicitly pass -Wno-unused-macros to gcc
• remove the macro from the source file and pass -D sscanf=THINK_TWICE_ABOUT_USING_SSCANF to gcc