* 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`.