* lisp/files.el: Don't allow customization of dir-locals sorting

* lisp/files.el: Don't allow customization of dir-locals sorting

In retrospect, this is not a good idea for the same reason that
`dir-locals-file' is a defconst, because it is important that this
behaviour be "uniform across different environments and users".
Sure, the user can still change the sorting with a hack, but we
shouldn't encourage them to change it.

(dir-locals--all-files): Return list in the order returned by
`file-expand-wildcards'.
(file-expand-wildcards): Document the sorting predicate used.
(dir-locals-sort-predicate): Delete variable.