Simplify mwheel-mode by using alist instead of two variables

Simplify mwheel-mode by using alist instead of two variables

* lisp/mwheel.el (mouse-wheel--remove-bindings): Update call
signature to take no arguments.  Doc fix.
(mouse-wheel--add-binding): Break out from...
(mouse-wheel-mode): ...here.  Simplify by using above functions.
(mouse-wheel--installed-bindings-alist): New variable.
(mwheel-installed-bindings): Make obsolete.
(mwheel-installed-text-scale-bindings): Make obsolete.
* test/lisp/mwheel-tests.el (mwheel-test-enable/disable):
New test.