todo-mode.el: Don't let-bind `buffer-read-only`

todo-mode.el: Don't let-bind `buffer-read-only`

Prefer let-binding `inhibit-read-only` so the code can freely change
`buffer-read-only`.  While at it, prefer #' to quote function names.

* lisp/calendar/todo-mode.el (todo-rename-category)
(todo-delete-category, todo-delete-item)
(todo-edit-item--diary-inclusion, todo-edit-category-diary-inclusion)
(todo-edit-category-diary-nonmarking, todo-archive-done-item)
(todo-unarchive-items, todo-display-categories)
(todo-update-categories-display, todo-filter-items-1)
(todo-reset-nondiary-marker, todo-reset-done-separator-string)
(todo-reset-done-string, todo-reset-comment-string):
Bind `inhibit-read-only` instead of `buffer-read-only`.
(todo-mode, todo-archive-mode, todo-edit-mode, todo-categories-mode)
(todo-filtered-items-mode): Let `define-derived-mode` take care of
adding the keymap to the docstring.
(todo-mode, todo-archive-mode, todo-categories-mode)
(todo-filtered-items-mode): Let `define-derived-mode` set
`mode-class`.