mwheel.el: Remove `mouse-wheel-*-alternate-event` vars

mwheel.el: Remove `mouse-wheel-*-alternate-event` vars

Now that `wheel-DIR` events are hardcoded, we never need more than
one variable (which we actually never needed anyway, we could have
let `mouse-wheel-*-event` vars hold lists of events instead), so
remove the `mouse-wheel-*-alternate-event` vars by merging their
default value into that of the corresponding `mouse-wheel-*-event`.

* lisp/mwheel.el (mouse-wheel-down-event, mouse-wheel-up-event)
(mouse-wheel-left-event, mouse-wheel-right-event): Don't bother holding
`wheel-DIR` events since these are already handled anyway.
Hold the event that would have been held in
`mouse-wheel-DIR-alternate-event` instead.
(mouse-wheel-down-alternate-event, mouse-wheel-up-alternate-event)
(mouse-wheel-left-alternate-event, mouse-wheel-right-alternate-event):
Delete vars.
(mwheel--is-dir-p, mouse-wheel--setup-bindings):
* lisp/edmacro.el (edmacro-fix-menu-commands):
* lisp/completion-preview.el (completion-preview--mouse-map):
Don't use `mouse-wheel-up/down-alternate-event` any more.
* lisp/progmodes/flymake.el (flymake--mode-line-counter-map):
Do nothing, because it already ignored those vars.

(cherry picked from commit 18294854c717a82966090e99130bcb99fc354a5b)