op.c:S_my_kid: Remove obsolete comment

/* XXX does this let anything illegal in? */

The commit that added that comment indeed allowed ‘our $$x’ and even
‘my $$x’, so it was a problem at the time.  It was this commit:

commit 77ca0c9
Author: Larry Wall <larry@wall.org>
Date:   Fri Sep 24 14:59:37 1999 -0700

    Re: [PATCH 5.005_61] "our" declarations

This commit fixed the problem:

commit b6512f4
Author: Mark-Jason Dominus <mjd@plover.com>
Date:   Mon Dec 24 13:14:48 2001 -0500

    PATCH: Restore "Can't declare scalar dereference in my" error

so the comment is no longer relevant.

op.c

@@ -3668,7 +3668,7 @@ S_my_kid(pTHX_ OP *o, OP *attrs, OP **imopsp)
 	return o;
     } else if (type == OP_RV2SV ||	/* "our" declaration */
 	       type == OP_RV2AV ||
-	       type == OP_RV2HV) { /* XXX does this let anything illegal in? */
+	       type == OP_RV2HV) {
 	if (cUNOPo->op_first->op_type != OP_GV) { /* MJD 20011224 */
 	    S_cant_declare(aTHX_ o);
 	} else if (attrs) {