doc/ctf-spec.texi: Remove "@validatemenus off"

Remove @validatemenus from ctf-spec.texi, which has been removed from
texinfo by

commit a16dd1a9ece08568a1980b9a65a3a9090717997f
Author: Gavin Smith <gavinsmith0123@gmail.com>
Date:   Mon Oct 12 16:32:37 2020 +0100

    * doc/texinfo.texi
    (Writing a Menu, Customization Variables for @-Commands)
    (Command List),
    * doc/refcard/txirefcard.tex
    Remove @validatemenus.
    * tp/Texinfo/XS/Makefile.am (command_ids.h): Use gawk instead
    of awk.  Avoid discouraged "$p" usage, using "$(p)" instead.
    * tp/Texinfo/XS/configure.ac: Check for gawk.

commit 128acab3889b51809dc3bd3c6c74b61d13f7f5f4
Author: Gavin Smith <gavinsmith0123@gmail.com>
Date:   Thu Jan 3 14:51:53 2019 +0000

    Update refcard.

    * doc/refcard/txirefcard.tex: @setfilename is no longer
    mandatory.  Do not mention @validatemenus or explicitly giving
    @node pointers, as these are not very important features.

	PR libctf/28567
	* doc/ctf-spec.texi: Remove "@validatemenus off".