Remove unnecessary exit(0);

This was necessary to avoid warnings in a specific compiler but after
adding MZ_DECLARE_NORETURN, this is now unnecessary.

racket/src/racket/src/error.c

@@ -4412,8 +4412,6 @@ scheme_raise_exn(int id, ...)
 #else
   call_error(buffer, alen, scheme_false);
 #endif
-
-  exit(0); /* should not get here, but makes NORETURN clearly true */
 }
 
 #ifndef NO_SCHEME_EXNS