Mark nnheader-cancel-timer as obsolete and adjust callers

Mark nnheader-cancel-timer as obsolete and adjust callers

* lisp/gnus/nnheader.el (nnheader-cancel-timer)
(nnheader-cancel-function-timers): Mark as obsolete.

* lisp/gnus/nntp.el (nntp-with-open-group-function)
(nntp-async-stop): Adjust caller.

* lisp/gnus/gnus-art.el (gnus-stop-date-timer): Ditto.

* lisp/gnus/gnus-async.el (gnus-async-prefetch-next): Ditto.

* lisp/gnus/gnus-demon.el (gnus-demon-run-callback)
(gnus-demon-cancel): Ditto.

* lisp/gnus/mail-source.el (mail-source-report-new-mail): Ditto.

* lisp/gnus/nnmaildir.el (nnmaildir-request-accept-article): Ditto.