mc_pos_control: remove useless landed state copy
mc_pos_control: remove useless landed state copy

If the landed state was set before the position controller starts it
would get updated with the first poll_subscriptions() anyways. But it's
not on all posix and nuttx platforms I tested and the deleted statement
just copies the zero message into the struct which is already zero.

mje-nz/PX4-Firmware,Aerotenna/Firmware,dagar/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,acfloria/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,dagar/Firmware,dagar/Firmware,krbeverx/Firmware,dagar/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,Aerotenna/Firmware,PX4/Firmware,acfloria/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,Aerotenna/Firmware