Remove index free function

Avoids including function in C++ side that resulted in OSS C++ errors:

include/mlir-c/Core.h:228:16: error: functions that differ only in their
return type cannot be overloaded
edsc_indexed_t index(edsc_indexed_t indexed, edsc_expr_list_t indices);
~~~~~~~~~~~~~~ ^
/usr/include/string.h:484:14: note: previous declaration is here
extern char *index (const char *__s, int __c)

And as these are going away soon, just removing the function requires the least changes.

PiperOrigin-RevId: 239110470