[Gnus]: Use overlay functions directly

[Gnus]: Use overlay functions directly

* lisp/gnus/gnus-art.el (gnus-mime-inline-part, gnus-mm-display-part)
(gnus-insert-mime-button, gnus-mime-buttonize-attachments-in-header)
(gnus-article-highlight-signature, gnus-article-extend-url-button)
(gnus-article-add-button, gnus-insert-prev-page-button)
(gnus-insert-next-page-button, gnus-insert-mime-security-button):
* lisp/gnus/gnus-cite.el (gnus-cite-delete-overlays)
(gnus-cite-add-face):
* lisp/gnus/gnus-html.el (gnus-html-wash-tags):
* lisp/gnus/gnus-salt.el (gnus-tree-read-summary-keys)
(gnus-tree-recenter, gnus-highlight-selected-tree):
* lisp/gnus/gnus-sum.el (gnus-summary-show-all-threads)
(gnus-summary-show-thread, gnus-summary-hide-thread)
(gnus-highlight-selected-summary):
* lisp/gnus/gnus-util.el (gnus-put-overlay-excluding-newlines):
* lisp/gnus/message.el (message-fix-before-sending)
(message-toggle-image-thumbnails):
* lisp/gnus/mm-decode.el (mm-convert-shr-links):
* lisp/gnus/sieve.el (sieve-highlight, sieve-insert-scripts):
Use overlay functions directly instead of using gnus-overlay-*,
message-overlay-*, and sieve-overlay-*.

* lisp/gnus/gnus-sum.el (gnus-remove-overlays):
* lisp/gnus/gnus.el (gnus-make-overlay, gnus-copy-overlay)
(gnus-delete-overlay, gnus-overlay-get, gnus-overlay-put)
(gnus-move-overlay, gnus-overlay-buffer, gnus-overlay-start)
(gnus-overlay-end, gnus-overlays-at, gnus-overlays-in):
* lisp/gnus/message.el (message-delete-overlay, message-make-overlay)
(message-overlay-get, message-overlay-put, message-overlays-in):
* lisp/gnus/sieve.el (sieve-make-overlay, sieve-overlay-put)
(sieve-overlays-at):
Remove.