Convert user_api header and user_init source files to valid C code.

PiperOrigin-RevId: 629691192
Change-Id: I9be78aca27ae584fff0b733e9015b2bb3b927ca4

src/user/user_util.cc

@@ -33,10 +33,6 @@ using std::string;
 using std::numeric_limits;
 
 
-// set value of NAN here; needs <limits>
-const double mjNAN = numeric_limits<double>::quiet_NaN();
-
-
 // check if numeric variable is defined
 bool mjuu_defined(const double num) {
   return !isnan(num);