Remove OPpCONST_WARNING

Remove OPpCONST_WARNING

This was added to op.h in commit 599cee73:

commit 599cee73f2261c5e09cde7ceba3f9a896989e117
Author: Paul Marquess <paul.marquess@btinternet.com>
Date:   Wed Jul 29 10:28:45 1998 +0100

    lexical warnings; tweaks to places that didn't apply correctly
    	Message-Id: <9807290828.AA26286@claudius.bfsec.bt.co.uk>
    	Subject: lexical warnings patch for 5.005_50

    p4raw-id: //depot/perl@1773

dump.c was modified to dump in, in this commit:

commit bf91b999b25fa75a3ef7a327742929592a2e7e9c
Author: Simon Cozens <simon@netthink.co.uk>
Date:   Sun May 13 21:20:36 2001 +0100

    Op private flags
    Message-ID: <20010513202036.A21896@netthink.co.uk>

    p4raw-id: //depot/perl@10117

But is apparently completely unused anywhere.  And I want that bit.