Revert "Fallbacks for Perl_fp_class_denorm()."

This reverts commit e77299d.

This was reverted due to a major freeze. It was merged into the
blead-next branch and will appear in 5.26.1.

perl.h

@@ -6867,26 +6867,6 @@ extern void moncontrol(int);
 #  endif
 #endif
 
-/* We have somehow managed not to define the denormal/subnormal
- * detection.
- *
- * This may happen if the compiler doesn't expose the C99 math like
- * the fpclassify() without some special switches.  Perl tries to
- * stay C89, so for example -std=c99 is not an option.
- *
- * The Perl_isinf() and Perl_isnan() should have been defined even if
- * the C99 isinf() and isnan() are unavailable, and the NV_MIN becomes
- * from the C89 DBL_MIN or moral equivalent. */
-#if !defined(Perl_fp_class_denorm) && defined(Perl_isinf) && defined(Perl_isnan) && defined(NV_MIN)
-#  define Perl_fp_class_denorm(x) ((x) != 0.0 && !Perl_isinf(x) && !Perl_isnan(x) && PERL_ABS(x) < NV_MIN)
-#endif
-
-/* This is not a great fallback: subnormals tests will fail,
- * but at least Perl will link and 99.999% of tests will work. */
-#if !defined(Perl_fp_class_denorm)
-#  define Perl_fp_class_denorm(x) FALSE
-#endif
-
 #ifdef DOUBLE_IS_IEEE_FORMAT
 #  define DOUBLE_HAS_INF
 #  define DOUBLE_HAS_NAN