title: Remove validation in angular 4 when it's not render in NgIf
body: <p>I want to remove validation for which control is not rendered by using NgIf. I was try to use directive to remove with hidden control but cannot do the same because it not render in template. So I can not check formControlName with ElementRef in directive. Here is ts file</p>
  
  <pre><code>this.form = this._fb.group({
    text1: ['', Validators.required],
    text2:  ['', Validators.required]
  });
  </code></pre>
  
  <p>and template </p>
  
  <pre><code>&lt;form[formGroup]="form"&gt;
    &lt;input type="text" formControlName="text1"&gt;
   &lt;div *ngIf="false"&gt;
    &lt;input type="text" formControlName="text2"&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>
  I want to remove Validation of text2 dynamically and globally. Not remove validator in ts file. </p>