regcomp.c: Remove extraneous conditional

This is checking that the argument is an ASCII \w that isn't an
underscore.  But in this range, that is identical to an alphanumeric,
which there is already a macro for, so use that single macro instead of
the two.

regcomp.c

@@ -10213,7 +10213,7 @@ S_regclass(pTHX_ RExC_state_t *pRExC_state, I32 *flagp, U32 depth,
                 }
             default:
                 /* Allow \_ to not give an error */
-                if (isWORDCHAR(value) && value != '_') {
+                if (isALPHANUMERIC_A(value)) {
                     if (strict) {
                         vFAIL2("Unrecognized escape \\%c in character class",
                                (int)value);