lisp/gnus/.dir-locals.el shadows top-level .dir-locals.el

[Glenn Morris]

lisp/gnus has this vital .dir-locals.el:

  ((emacs-lisp-mode . ((show-trailing-whitespace . t))))

(Which seems like personal preference to me.)

Anyway, this prevents the actually-has-useful-content
(log-edit-rewrite-fixes etc) top-level Emacs .dir-locals.el from being
seen.

The simplest solution seems not to have lisp/gnus/.dir-locals.el in the
Emacs repo. (Rather than duplicating all the top-level content into the
Gnus one, or implementing .dir-locals inheritance, desirable though that
might be.)