This removes the weston-screensaver client.

Screensavers are not so useful, DPMS is much better. This example has
existed here for a good while, and things that we could learn from it
have been learnt.

Nowadays this is just dead weigth, which is usually not even compiled,
because it depends on both cairo-gl and GLU. Removing it removes the
only possible dependency to GLU and one user of cairo-gl. Now the last
user of cairo-gl is gears (clients/nested.c uses cairo-glesv2).

Support for screensavers is still left in desktop-shell, so external
projects can still have their screensavers if they want.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Acked-by: Giulio Camuffo <giuliocamuffo@gmail.com>