Shadows annotation function for backends that don't use metadata

[mohkale]

Issue Description

Hi there,

Recently I discovered vertico actually renders annotation function content in the completion-at-point overlay provided by consult, like so:

However this doesn't work for eglot as a completion backend which is surprising.

After some back and forth I managed to track down this package as the cause 😞. After disabling it eglot annotations do show up in the vertico overlay.

Reproduction Instructions

1. Bring up a emacs -q instance and run the following.

(progn
  (setq package-archives
        '(("MELPA" . "https://melpa.org/packages/")
          ("gnu" . "http://elpa.gnu.org/packages/")))
  (package-refresh-contents)
  (use-package eglot
    :demand t)
  (use-package all-the-icons-completion
    :ensure t
    :hook (marginalia-mode . all-the-icons-completion-marginalia-setup))
  (use-package marginalia
    :ensure t
    :hook (vertico-mode . marginalia-mode))
  (use-package vertico
    :demand t
    :ensure t
    :config
    (vertico-mode))
  (use-package consult
    :ensure t
    :demand t
    :config
    (setq completion-in-region-function #'consult-completion-in-region)))

2. Find a C++ project and enable eglot in it. here's an example project.
3. Open a CPP file and try inserting something.
4. Run M-x completion-at-point and get completions without annotations.
5. Run M-x all-the-icons-completion-mode to disable this package.
6. Run M-x completion-at-point and get completions with annotations.

Cause of Bug

Seems to be because eglots capf doesn't set an annotation function, it sets a :annotation-function property in the capf which consult re-exports in completion-extra-properties but this package ignores and then sets a annotation function just to set an icon and thus overrides.