title: Remove / Replace the username field with email using FOSUserBundle in Symfony2 / Symfony3
body: <p>I only want to have email as mode of login, I don't want to have username. Is it possible with symfony2/symfony3 and FOSUserbundle?</p>
  
  <p>I read here <a href="http://groups.google.com/group/symfony2/browse_thread/thread/92ac92eb18b423fe" rel="noreferrer">http://groups.google.com/group/symfony2/browse_thread/thread/92ac92eb18b423fe</a></p>
  
  <p>But then I am stuck with two constraint violations.</p>
  
  <p>Problem is if the user leaves the email address blank, I get two constraint
  violations:</p>
  
  <ul>
  <li>Please enter a username</li>
  <li>Please enter an email</li>
  </ul>
  
  <p>Is there a way to disable validation for a given field, or a better way to
  remove a field from the form altogether? </p>