Is it safe to delete /private/var/sleepimage on OS X?
You can safely delete it without going into Target Disk Mode.
sudo rm /private/var/vm/sleepimage
Note that this will be recreated when you sleep your computer again. To disable safe sleep altogether (something I don't recommend),
sudo pmset -a hibernatemode 0
sudo rm /private/var/vm/sleepimage
Doing this may cause you to lose data if your computer loses power while sleeping.