* lisp/gnus/gnus-art.el: Fix up compiler warnings.

(article-display-face, article-display-x-face): Remove unused `face'.
(gnus-article-browse-html-save-cid-content): Remove unused var `type'.
(article-date-ut): Remove unused var `first'.
(gnus-article-prepare): Remove unused var `gnus-article'.
(gnus-mime-save-part-and-strip): Remove unused var `param'.
(gnus-mime-inline-part): Remove unused vars `charset', `contents', and
`coding-system' along with corresponding dead code.
(gnus-mime-view-part-externally): Remove unused var
`mm-user-display-methods'.
(gnus-insert-mime-button): Let-bind gnus-tmp-id explicitly.
(gnus-display-mime): Remove unused var `handle'.
(gnus-mime-display-alternative): Remove unused var `props'.
(gnus-article-read-summary-keys): Remove unused var `up-to-top'.
(gnus-article-edit-done): Remove unused var `p'.
(gnus-url-mailto): Remove unused var `to'.
(gnus-treat-article): Let-bind gnus-treat-condition, part-number,
total-parts, and gnus-treat-type explicitly.  Remove unused var `elem'.

* lisp/gnus/mm-util.el (mm-with-unibyte-current-buffer): Mark obsolete and
add warning.