Follow-up from "Make weston_buffer useful [weston_buffer epic part 2]"

The following discussion from !767 (merged) (really it was part of !766 (merged) so it already got merged ...) should be addressed:

• @leandrohrb started a discussion: (+1 comment)

  At this point you can drop surface from struct focus_surface and also remove weston_surface_destroy(fsurf->surface) from focus_surface_destroy().

  This change would make shell: Encapsulate weston_curtain in its own struct clearer.