`define-widget-keywords' has been a no-op macro since 1999.

I suggest that we mark it as obsolete.

Please refer to this commit for more information:
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=b3e49cbb9cbb8697a669cc4dbdb60c8e3a1b7a5c

Thanks,
Stefan Kangas