whitespace.el: Collaborate better with `combine-after-change-calls`

whitespace.el: Collaborate better with `combine-after-change-calls`

* lisp/whitespace.el: Remove redundant `:group` arguments.
Prefer #' to quote function names.
(whitespace-buffer-changed): Delete function.
(whitespace-color-on): Don't touch `before-change-functions`.
(whitespace--update-bob-eob): Set `whitespace-buffer-changed` here, instead.
(whitespace-post-command-hook): Apply De Morgan.
(whitespace--empty-at-bob-matcher, whitespace-post-command-hook)
(whitespace--update-bob-eob): Use `point-min` and `point-max`.

(cherry picked from commit 7cb7e96a5cca6778054cf74fd2c7548ca40f0691)