* mime-edit.el: Don't require `invisible'.

* mime-edit.el: Don't require `invisible'.
(mime-edit-insert-voice): Don't use `mime-edit-invisible-region'.
(mime-edit-insert-binary-file): Ditto.
(mime-edit-content-end): Don't use `mime-edit-invisible-p' and
`mime-edit-next-visible-point'.
(mime-edit-invisible-region): Abolish.
(mime-edit-invisible-p): Abolish.
(mime-edit-next-visible-point): Abolish.
(turn-on-mime-edit): Don't call `enable-invisible'.
(mime-edit-exit): Don't call `end-of-invisible'.