title: how to cleanly remove ndb properties
body: <p>in my app i need to remove a few of my models properties.<br>
  i checked out <a href="https://developers.google.com/appengine/articles/update_schema" rel="noreferrer">this link</a> but the first issue is that the properties are on a <code>polymodel</code> and there is no way im going to switch to an <code>expando</code> for the time to remove the properties, im not even shure what could happen if i change a <code>polymodel</code> to an <code>expando</code>.</p>
  
  <p>so how do i remove properties from existing entities?</p>
  
  <p>i was thinking to set all <code>StringProperty</code> to <code>None</code> and then remove these from the model schema and redeploy. 
  one of those properties is a <code>BooleanProperty</code>, i can't set this one to <code>None</code> right?!
  or an <code>ndb.PickleProperty</code>... how should i remove that?</p>
  
  <p>does anybody know how to get this done properly?</p>