This reverts commit b6d8efbe.

This GLib API is good, but the implentation is not ready, so there's no
reason to commit to the API in GLib 2.64. We can reland again when the
implementation is ready.

There are three problems: (a) The glib-networking implementation normally
works, but the test has been broken for a long time. I'm not comfortable
with adding a major new feature without a working test. This is
glib-networking#104. (b) The WebKit implementation never landed. There
is a working patch, but it hasn't been accepted upstream yet. This API
isn't needed in GLib until WebKit is ready to start using it.
https://bugs.webkit.org/show_bug.cgi?id=200805. (c) Similarly, even if
the WebKit API was ready, that itself isn't useful until an application
is ready to start using it, and the Epiphany level work never happened.

Let's try again for GLib 2.66. Reverting this commit now just means we
gain another si...