Edit: I believe this was removed because the kernal already contained another combo that does the same job, Alt+SysReq+k will terminate and restart X if it should become unresponsive.
If you would prefer the old key combination then issue the following two commands in a terminal. I would advise against this though, but as always, the choice is entirely yours.
sudo apt-get install dontzapthen do:
sudo dontzap –disable