sitl_run.sh: do not kill jmavsim if model is none
sitl_run.sh: do not kill jmavsim if model is none

I sometimes start jmavsim independently from px4, and in that case jmavsim
should not be killed. These are the commands:
./Tools/jmavsim_run.sh
make posix none

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