staging: pi433: remove unused rf69_set_ook_threshold_step function

Function rf69_set_ook_threshold_step is unused and should be removed
along with type enum thresholdStep 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>