* lisp/mail/footnote.el: Minor simplifications

* lisp/mail/footnote.el: Minor simplifications

Remove redundant :group args.
(footnote-mode-hook): Let define-minor-mode define it.
(footnote--style-p): Delete function.
(footnote--index-to-string): Inline it instead, and simplify.
(footnote-cycle-style): Use a pointer into the alist as the "index"
instead of a number.
(footnote-set-style): Use footnote-style-alist as the completion table.
Prefer `assq` over `footnote--assoc-index`.
(footnote--assoc-index): Delete function.
(footnote--renumber): Remove first (unused) argument; Adjust all callers.
(footnote--sort): Use car-less-than-car.