Delete support for the Conkeror web browser

Delete support for the Conkeror web browser

The last release of Conkeror was in 2017.  The project has been actively
recommending people not to use it since 2019 due to security concerns,
and it was removed from Debian in the same year.  Emacs's browse-url
support for Conkeror has been obsolete since in Emacs 28.1.

* lisp/net/browse-url.el (browse-url-conkeror-new-window-is-buffer):
(browse-url-conkeror-program, browse-url-conkeror-arguments): Delete
variables.
(browse-url-conkeror): Delete function.

(cherry picked from commit 4415102b573bb776e7f256633626f798b2ab9f28)