docs: remove devhelp API docs

We currently generate two completely separate API references for the
libvirt public API. One at 'docs/html/' and one at 'docs/devhelp/'.
Both are published on the website, but we only link to content in
the 'docs/html/' pages.

Both are installed in the libvirt-docs sub-RPM, with a full copy
of the website including 'docs/html/' in /usr/share/docs/libvirt-docs,
while the 'docs/devhelp/' content goes to /usr/share/gtk-doc/. The
latter was broken for years until:

commit ca6f602546cb28658db05f29bc840e04d22d0947
Author: Andrea Bolognani <abologna@redhat.com>
Date: Fri May 10 14:54:52 2019 +0200

docs: Introduce $(devhelphtml_generated)

Our XSLT magic generates one Devhelp-compatible HTML file
per documentation module, but so far we have only shipped
and installed documentation for virterror.

Now that we have $(modules), however, we can generate the
list of files the same way we do for regular documentation
and make sure we always ship and install everything.

That this bug went unnoticed for so long is a sign of how few
people are using the devhelp docs. The only commits to the devhelp
code since it was first introduced have been fixing various build
problems that hit.

The only obvious difference between the two sets of docs is the CSS
styling in use. Overall devhelp does not look compelling enough to
justify having two duplicated sets of API docs. Eliminating it will
reduce the amount of XSL code we are carrying in the tree which is
an attractive benefit.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>