wayland-server: Finally remove deprecated struct wl_buffer definition

commit d94a8722
warned this was coming, back in 2013.

I've seen libraries that have wayland client and server using functions
in the same file.  Since struct wl_buffer still exists as an opaque
entity in client code, the vestigial deprecated wl_buffer from the
server include will generate warnings when not building with

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
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