mc_att_control: remove useless parameter MC_YAW_FF

It scales the yawspeed setpoint arbitrarily by default with 0.5.
This makes no sense because when you give a setpoint of 1rad/s then
you expect the setpoint to get executed. If you want manual yawspeed
response to be less agressive on the stick use the scaling parameter
for the stick MPC_MAN_Y_MAX.