Supress all automatic indentation in SGML mode

I use Gnu Emacs 1:30.1 to edit SGML files on Debian 13.  Whenever I press 
"Enter" for a new line, the Emacs SGML mode adds two spaces. I find this useless 
and irritating.  I would like to suppress all automatic indentation in SGML 
mode.

I am not a Lisp programmer, so I need a working snippet that I can add to my 
.emacs .  I copied this code from a FreeBSD site :

(setq sgml-mode-hook
    '(lambda ()
       (setq fill-column 70
             indent-tabs-mode nil
             next-line-add-newlines nil
             standard-indent 0)
       (auto-fill-mode t)))
     
but it has no effect.

Any snippet which does work would be gratefully received.  Thanks, Roger