fix(Effects): Remove toPayload utility function (#738)

BREAKING CHANGE: The utility function `toPayload`, deprecated in @ngrx/effects v4.0, has been removed.

   Before:
    
    ```ts
    import { toPayload } from '@ngrx/effects';
    
    actions$.ofType('SOME_ACTION').map(toPayload);
    ```

    After:

    ```ts
    actions$.ofType('SOME_ACTION').map((action: SomeActionWithPayload) => action.payload)
    ```

modules/effects/src/index.ts

@@ -8,6 +8,5 @@ export { Actions, ofType } from './actions';
 export { EffectsModule } from './effects_module';
 export { EffectSources } from './effect_sources';
 export { OnRunEffects } from './on_run_effects';
-export { toPayload } from './util';
 export { EffectNotification } from './effect_notification';
 export { ROOT_EFFECTS_INIT } from './effects_root_module';