Now that "The Champion is X" msg is centerprinted only once there's no need to strzon...

Now that "The Champion is X" msg is centerprinted only once there's no need to strzone champion.netname. Also I had to fix other centerprint-related bugs in the arena code (yet another couple of bugs created BY SOMEONE ELSE (guess who?) and fixed BY ME):
* "The Champion is X" msg was never sent because champion was cleared just before sending it (bad check f > 0).
Furthermore, bug in the bug, although the champion is needed only in arena, mysteriously its name was strzoned only in a part of the ca code
* "Begin!" msg in arena mode was never sent cause of a missing FOR_EACH_PLAYER(e)