AC_MotorsHeli: modified throttle curve

Replace THROTTLE_MIN and HIGH with RC8_MIN and RC8_MAX
Shift RSC_CURVE_IDLE and LOW param range from 0 to 1000 and remove
write_rsc_pwm method
Move throttle curve calcs to new set_desired_rotor_speed_from_collective
method
TradHeli: use motors set_desired_rotor_speed_from_collective