docs: Remove checks for whether an effect is disabled

pre_paint() and post_paint() implementations don't need
to check whether an effect is disabled: Clutter will
not apply an effect unless it is enabled.

So remove code which checks whether the effect is
enabled or disabled from the example applications and the
documentation.

doc/cookbook/examples/cb-border-effect.c

@@ -35,12 +35,6 @@ cb_border_effect_post_paint (ClutterEffect *self)
   gfloat height;
   CbBorderEffectPrivate *priv;
 
-  ClutterActorMeta *meta = CLUTTER_ACTOR_META (self);
-
-  /* check that the effect is enabled before applying it */
-  if (!clutter_actor_meta_get_enabled (meta))
-    return;
-
   priv = CB_BORDER_EFFECT (self)->priv;
 
   /* get the associated actor's dimensions */