* lisp/completion-preview.el: Fix use in non-GUI session

* lisp/completion-preview.el: Fix use in non-GUI session

Fix loading in non-GUI sessions where `mwheel` is not preloaded.
Not requiring `mwheel` would be a lot more complex, since it would
require delaying the construction of `completion-preview--mouse-map`.

* lisp/completion-preview.el (<toplevel>): Require `mwheel`.
Remove correspondingly redundant `defvar`s.
(completion-preview--mouse-map): Use `key-description` rather than mimicking
it with `format`.

(cherry picked from commit 82f71e106afd9bede95cfea3025f7c059d7c2bcf)