compositor: Remove unused weston_surface_to_buffer function

The function 'weston_surface_to_buffer' is unused by compositor and
clients inside weston, so it should be safe to remove this function

Signed-off-by: Chris Michael <cp.michael@samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
2 files changed