Remove an assertion in EvalClock that was forbidding expression without output clock.

But actually, it is possible to have node without output, so expression
without output clock is perfrectly possible, as in the following example :

node h(x : bool) returns ();
 var h : bool;
let
  h = false fby (not x);
tel

node f(x : bool) returns (y: bool);
var
  u:bool;
let
  y,u = (false, h(true), true) fby (true, x, h(false));
tel