op.c:S_my_kid: Remove obsolete comment

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 77ca0c92d2c0e47301d906d355d9ab3afb6f6bcb
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 b6512f489e761186d508cf0b8b7705805cfefc52
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.