Simplify the code for using dired to attach messages

I originally sent some code to the mailing list in 2012 to use dired
for marking attachments.

https://groups.google.com/g/mu-discuss/c/OPwdNZbB5GE/m/hnjNlNoLIu8J

As the function in gnus.el has since been fixed, the version of
gnus-dired-mail-buffers now works fine with mu4e (and has done for
many years).  I suggest deleting the old code to make the
documentation simpler assuming that people are no longer on Emacs from
9 years ago.

mu4e/mu4e.texi

@@ -3786,26 +3786,8 @@ It is possible to attach files to @t{mu4e} messages using @t{dired}
 (@inforef{Dired,,emacs}), using the following steps (based on a post on the
 @t{mu-discuss} mailing list by @emph{Stephen Eglen}).
 
-To prepare for this, you need a special version of the
-@code{gnus-dired-mail-buffers} function so it understands @t{mu4e} buffers as
-well; so put in your configuration:
 
 @lisp
-(require 'gnus-dired)
-;; make the `gnus-dired-mail-buffers' function also work on
-;; message-mode derived modes, such as mu4e-compose-mode
-(defun gnus-dired-mail-buffers ()
-  "Return a list of active message buffers."
-  (let (buffers)
-    (save-current-buffer
-      (dolist (buffer (buffer-list t))
-        (set-buffer buffer)
-        (when (and (derived-mode-p 'message-mode)
-                (null message-sent-message-via))
-          (push (buffer-name buffer) buffers))))
-    (nreverse buffers)))
-
-(setq gnus-dired-mail-mode 'mu4e-user-agent)
 (add-hook 'dired-mode-hook 'turn-on-gnus-dired-mode)
 @end lisp