message: remove experimental volume kernel CMake option
diff: ## limitations under the License. ##
  ## ======================================================================== ##
  -option(EXP_NEW_BB_VOLUME_KERNELS
  - "Experimental new block/bricked volume layout" ON)
  -mark_as_advanced(EXP_NEW_BB_VOLUME_KERNELS)
  -
  ospray_build_component(ospcommon)
  # -------------------------------------------------------
  