commander: remove unused HITL transition function

To my knowledge this hil transition function is not used anymore,
however, it makes sending the DO_SET_MODE command unnecessarily complex.
In my opinion the DO_SET_MODE command should only change the mode but
not other things like arming (already removed) and HITL state (this
commit).

Often times, I was seeing the error message "Set SYS_HITL to 1 and
reboot to enable HITL." when using QGC with a vehicle in HITL.

HITL is set via parameter which then has an impact in the startup script
where the CLI argument `-hil` is added to some of the commands that
require it (like commander as well).