Remove definition of ISDIGIT_LOCALE(c)

It wasn't being used at all.  Let's remove it.

Use isdigit(3) directly in comments that referenced it.

Also, in those comments, remove an outdated reference to the fact
that ISDIGIT_LOCALE(c) might evaluate its argument more than once,
which could be true a few commits ago, until
IN_CTYPE_DEFINITION(c) was removed.  Previously, the definition
for ISDIGIT_LOCALE(c) was:

 #if defined (STDC_HEADERS) || (!defined (isascii) && !defined (HAVE_ISASCII))
 # define IN_CTYPE_DOMAIN(c) 1
 #else
 # define IN_CTYPE_DOMAIN(c) isascii(c)
 #endif

 #define ISDIGIT_LOCALE(c) (IN_CTYPE_DOMAIN (c) && isdigit (c))

Which could evaluate 'c' twice on pre-C89 systems (which I hope
don't exist nowadays).

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>

lib/defines.h

@@ -22,8 +22,6 @@ typedef unsigned char _Bool;
 # define __bool_true_false_are_defined 1
 #endif
 
-#define ISDIGIT_LOCALE(c) isdigit (c)
-
 /* Take care of NLS matters.  */
 #ifdef S_SPLINT_S
 extern char *setlocale(int categories, const char *locale);