mwheel.el: Unconditionally use the `wheel-up/down/...` events

mwheel.el: Unconditionally use the `wheel-up/down/...` events

The `mouse-wheel-DIR-event` vars were introduced because under X11
we get different `mouse-N` events depending on the users' mouse and
those same events can be used for other things for other rodents, so we
can't unconditionally treat those events as mouse-wheel events.

But this does not apply to the `wheel-up/down/...` events.
So hard code them.

* lisp/mwheel.el (mwheel--is-dir-p): Always consider the `wheel-DIR` events.
(mouse-wheel--setup-bindings): Always bind the `wheel-DIR` events.

* lisp/completion-preview.el (completion-preview--mouse-map):
Unconditionally bind the `wheel-DIR` events.
* lisp/edmacro.el (edmacro-fix-menu-commands): Hard code the
`wheel-DIR` events as mouse events regardless of `mouse-wheel-*-event`s.
* lisp/progmodes/flymake.el (flymake--mode-line-counter-map):
Do nothing, because it's already been done in commit e5be6c7ae309.

* doc/lispref/commands.texi (Misc Events): Document the need to use
`wheel-up/down/left/right` unconditionally.