(highlight-changes-mode): Removed references to hooks that no longer exist.

(highlight-changes-mode): Removed references to hooks that no longer exist.
Because define-minor-mode is used the hook highlight-changes-mode-hook exists
and can do what both the old hooks used to do.  The documentation at the top
of the file was updated to demonstrate this.
(highlight-changes-mode): Remove commented out call to a hook function that
is no longer there.
(hilit-chg-set): Remove running of highlight-changes-enable-hook.