remove a bunch of XXX's from pad.c

remove a bunch of XXX's from pad.c

When in 2002 I moved a bunch of code from op.c etc into a new file,
pad.c, I left this comment at the top:

    /* XXX DAPM
     * As of Sept 2002, this file is new and may be in a state of flux for
     * a while. I've marked things I intent to come back and look at further
     * with an 'XXX DAPM' comment.
     */

Well, 12 years have passed since then, and if I was going to do any of
this stuff I would probably have done it by now, or someone else would.

So this commit removes the XXX's.