actors: Remove nesting of actions in define-simple-actor.

* 8sync/systems/actors.scm (define-simple-actor): Move actions out of
  the "final parenthesis".  Simple actors just have a class type and
  then the rest of the arguments are acitons.
* demos/actors/simplest-possible.scm: Remove unnecessary import
  of (ice-9 match).
  (<emo>, <proog>): Adjust to new syntax for define-simple-actor.