ON_DEBUG: delete unused macro

This is no longer used.

bignum.c

@@ -2914,30 +2914,6 @@ bary_divmod(BDIGIT *qds, size_t qn, BDIGIT *rds, size_t rn, const BDIGIT *xds, s
 # define BIGNUM_DEBUG (0+RUBY_DEBUG)
 #endif
 
-#if BIGNUM_DEBUG
-#define ON_DEBUG(x) do { x; } while (0)
-static void
-dump_bignum(VALUE x)
-{
-    long i;
-    printf("%c0x0", BIGNUM_SIGN(x) ? '+' : '-');
-    for (i = BIGNUM_LEN(x); i--; ) {
-        printf("_%0*"PRIxBDIGIT, SIZEOF_BDIGIT*2, BDIGITS(x)[i]);
-    }
-    printf(", len=%"PRIuSIZE, BIGNUM_LEN(x));
-    puts("");
-}
-
-static VALUE
-rb_big_dump(VALUE x)
-{
-    dump_bignum(x);
-    return x;
-}
-#else
-#define ON_DEBUG(x)
-#endif
-
 static int
 bigzero_p(VALUE x)
 {