Drop support for the dead third-party w3 package

Drop support for the dead third-party w3 package

The w3 package was removed from GNU ELPA in 2020 as it doesn't run on
a recent Emacs, and development had stopped over a decade before that.
If anyone wants to revive the w3 package, they should look this all
over, but it doesn't make sense for us to maintain this support code.
Ref: https://debbugs.gnu.org/25395

* lisp/net/browse-url.el (browse-url-w3): Make obsolete.
(browse-url--browser-defcustom-type)
(browse-url-default-browser):
* lisp/ffap.el (ffap-url-at-point, ffap-file-at-point):
* lisp/net/newst-plainview.el (newsticker--buffer-do-insert-text):
* lisp/net/newst-reader.el (newsticker-html-renderer)
(newsticker-show-news):
* lisp/net/newst-treeview.el (newsticker--treeview-render-text):
* lisp/org/ol.el (org-store-link):
* lisp/url/url.el (url-retrieve): Remove w3 support code and related
documentation and comments.  (Bug#56435)

* test/lisp/net/browse-url-tests.el
(browse-url-tests-browser-kind): Adjust test for above changes.

* etc/TODO: Remove TODO to install W3.
* doc/misc/org.org (Handling Links): Don't mention W3.
* lisp/msb.el (msb--few-menus, msb--very-many-menus): Check for
eww-mode instead of w3-mode.