staging: pi433: remove unused rf69_set_ook_threshold_type function

Function rf69_set_ook_threshold_type is unused and should be removed
along with type enum thresholdType which was used only by that function.

Signed-off-by: Marcin Ciupak <marcin.s.ciupak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>