Never send user email address in HTTP requests

Never send user email address in HTTP requests

It used to be possible to customize 'url-privacy-level' so that the
user's email address was sent along in HTTP requests.  Since
'url-privacy-level' is also a blocklist, rather than an allowlist,
this meant that a mere misconfiguration of Emacs risked exposing the
user's email address.  This is a serious privacy risk, and it is thus
better if we remove this dangerous feature altogether.

* lisp/url/url-http.el (url-http-create-request): Never send the
user email address.
* lisp/url/url-vars.el (url-personal-mail-address): Make obsolete.
* lisp/url/url-privacy.el (url-setup-privacy-info): Don't set
above obsolete variable.
* doc/misc/url.texi (Customization):
* lisp/url/url-vars.el (url-privacy-level): Update documentation
to reflect the above changes.