bug#59314: 29.0.50; EUDC and message-mode header completion

[Eric Abrahamsen]

Address completion in message-mode has stopped working in master,
possibly as a result of 0e25a39e69acca0324c326ea8e46b1725594bff5. This
has been reported for several contact-management backends that expect to
have their completions available with <TAB>.

`completion-at-point-functions' contains '(eudc-capf-complete
message-completion-function t) at this point -- `eudc-capf-complete'
returns no matches, and no other functions in the list are consulted.

On gnus.general, someone using BBDB and corfu reported that this recipe
fixed the problem:

  (setq eudc-server-hotlist '(("localhost" . bbdb)))

  (add-hook 'message-mode-hook
            (lambda ()
              (setq-local completion-at-point-functions
                          (delq 'message-completion-function
                                completion-at-point-functions))))

Someone else *not* using corfu reported that that didn't work for them.
Dunno.

Eric