bug#68213: 30.0.50; completion-preview-tests failure in --without-x build

[Eli Zaretskii <eliz@gnu.org>]

> From: Eshel Yaron <me@eshelyaron.com>
> Cc: jm@pub.pink,  68213@debbugs.gnu.org
> Date: Wed, 03 Jan 2024 19:45:39 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >   . does the problem happen only in a --without-x build?
> >   . what "duplicate" definitions do we have there and why?
> 
> The definition of `completion-preview--mouse-map` binds the events
> specified by both `mouse-wheel-up-event` and
> `mouse-wheel-up-alternate-event` to `completion-preview-prev-candidate`
> (using `defvar-keymap`).  When these two variables have the same value,
> as apparently happens in John's `--without-x` build but could probably
> also occur in other setups, `defvar-keymap` complains (signals an error)
> about a duplicate definition.  The duplication itself is harmless in
> this case, but there doesn't seem to be a way to tell `defvar-keymap`
> not to worry about it.

We should try not to produce duplicate bindings here.  Is there a real
possibility that these will be the same events in GUI builds?  If so,
the definition of bindings should detect that and avoid duplicate
bindings.  But if this can only happen in a --without-x build, then a
solution can be much simpler: avoid binding any mwheel events at all.
That's why I asked that first question.

In any case, I don't think this issue is significant enough to justify
any infrastructure changes in defvar-keymap.