lisp/editorconfig: Advise `find-auto-coding` i.s.o `insert-file-contents`

lisp/editorconfig: Advise `find-auto-coding` i.s.o `insert-file-contents`

In Emacs<30, we can't use `auto-coding-functions`, but we can get
almost the same result by advising `find-auto-coding`.

* lisp/editorconfig.el (editorconfig-merge-coding-systems): Return nil
if there's no setting.  Don't merge if there's no need.
(editorconfig-set-coding-system-revert); Adjust accordingly.
(editorconfig--filename-codingsystem-hash): Delete var.
(editorconfig--advice-insert-file-contents): Delete function.
(editorconfig--advice-find-file-noselect); Simplify.
(editorconfig--advice-find-auto-coding): New function.
(editorconfig-mode): Use it.