Remove local uniquify functions in favour of seq-uniq

Remove local uniquify functions in favour of seq-uniq

* lisp/emacs-lisp/seq.el (seq-uniq): Add autoload cookie.
* lisp/pcomplete.el: (pcomplete-uniquify-list): Use seq-uniq.
* lisp/eshell/esh-util.el (eshell-uniqify-list)
(eshell-uniquify-list):
* lisp/nxml/rng-util.el (rng-uniquify-equal):
* lisp/progmodes/idlwave.el (idlwave-uniquify):
* lisp/textmodes/artist.el (artist-uniq): Make into obsolete
function aliases for seq-uniq.  Update callers.
* lisp/nxml/rng-util.el (rng-uniquify-eq): Make obsolete in favor
of seq-uniq.  Update callers.