libweston: remove fbdev backend

Fbdev backend was deprecated in the Weston 10.0.0 release with
6338dbd5. Before that, I suggested
already in 2019 to remove it, but it was too soon then. Now it seems the
final voices asking for fbdev to be kept have been satisfied, see the
linked issue.

Fbdev-backend uses a kernel graphics UAPI (fbdev) which is sub-par for a
Wayland compositor: you cannot do GPU accelerated graphics in any
reasonable way, no hotplug support, multi-output support is tedious, and
so on. Most importantly, Linux has deprecated fbdev a long time ago due
to the UAPI fitting modern systems and use cases very poorly, but cannot
get rid of it if any users remain. Let's do here what we can to reduce
fbdev usage.

I am doing color management related additions to libweston which require
adding checks to every backend. One backend less is less churn to write
and review.

Libweston major version has already been bumped to 11, so the next
release will be Weston 11, without fbdev. enum weston_compositor_backend
entries change their numerical values.

Fixes: wayland/weston#581



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>