Game Events: removed assertion if weak event handler ptr fails to lock (fixup 056d7ac8f8)

I kinda assumed this should never happen due to the cleanup stage, but it seems there are
still cases where it does. Since the code already checks that the lock is valid before
returning, this isn't apparently needed and just breaks things.

src/game_events/manager.cpp

@@ -144,8 +144,6 @@ static handler_ptr lock_ptr(const handler_list& list, handler_list::iterator ite
 	if(iter != list.end()) {
 		if(handler_ptr ptr = iter->lock()) {
 			return ptr;
-		} else {
-			assert(false && "Found null handler in handler list!");
 		}
 	}