[3.13] Revert "gh-136006: fix Py_NAN expansion on Solaris systems (GH-136575) (#138734)" (#139239)

This reverts commit df34903 as discussed in #138733 (comment)

Author: epbennetts

Include/pymath.h

@@ -54,24 +54,9 @@
 
 /* Py_NAN: Value that evaluates to a quiet Not-a-Number (NaN).  The sign is
  * undefined and normally not relevant, but e.g. fixed for float("nan").
- *
- * Note: On Solaris, NAN is a function address, hence arithmetic is impossible.
- * For that reason, we instead use the built-in call if available or fallback
- * to a generic NaN computed from strtod() as a last resort.
- *
- * See https://github.com/python/cpython/issues/136006 for details.
  */
 #if !defined(Py_NAN)
-#  if defined(__sun)
-#    if _Py__has_builtin(__builtin_nanf)
-#       define Py_NAN   ((double)__builtin_nanf(""))
-#    else
-#       include <stdlib.h>
-#       define Py_NAN   (strtod("NAN", NULL))
-#    endif
-#  else
-#    define Py_NAN      ((double)NAN)
-#  endif
+#    define Py_NAN ((double)NAN)
 #endif
 
 #endif /* Py_PYMATH_H */