Here is a simple way to do it. First, make a trigger covering the whole mission area, activated by 'anybody present' (one time). In it's 'on activation' field, write:
{_x addeventhandler ["killed", {_this exec "killed.sqs"}]} foreach thislist
Then make a script titled "killed.sqs", and put the following inside of it:
~60
deletevehicle (_this select 0)
exit
The ~60 means that it will wait 60 seconds after the unit is killed, then it will delete it. If you want a shorter or longer delay, just change that number.