staging: vboxvideo: remove dead gamma lut code

The redundant fb helpers .load_lut, .gamma_set and .gamma_get are
no longer used. Remove the dead code that was not doing anything
sensible anyway.

Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170804104506.2414-1-peda@axentia.se