libweston: remove weston_output::head

Remove the scaffolding that allowed backends to be converted one by one
to the head-based API. Nothing is using these members anymore.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>