title: remove specified ttl in cassandra
body: <p>I read about updating ttl and that it is only possible by updating row.
  But I want to remove ttl. I fear it is the same process, but I did not found any information about it. Is there a way to remove ttl without updating all rows?</p>
  
  <p>What I do, is saving user information with ttl when user is registrating. So if the user do not validate his/her mail address the entry will automaticly delete. </p>