title: Angular Material: Remove item onDrop outside cdk component
body: <p>I am currently using angular material CDK DragAnDrop for some list functionalities. <br>What I'm trying to accomplish is:</p>
  
  <ul>
  <li><p>I drag an element from List A and drop it into List B.
  If I drag the element from List B and drop it outside List B (outside List A also) I would like to remove the element from List B.</p></li>
  <li><p>Is there some directive or api that I can use to validate that I'm dropping an element outside cdkDropList component?</p></li>
  </ul>