when using an external tail gyro on a flybar heli the stick input
should be directly passed to output. This patch fixes the use of
deadzone in that passthrough.

It also makes the tail handling consistent with roll and pitch
handling, by not using ACRO_YAW_P when in tail pass-through.

Finally it also fixes deadzone handling for roll and pitch, and
removes the unnecessary get_pilot_desired_yaw_rate() that has a
different prototype from the one used in the rest of the code