code: function deleteToken(username, token, done) {
    return tokenExists(username, token, function(existsErr, tokenObj) {
      if (existsErr) {
        return done(existsErr);
      }
  
      if (!tokenObj) {
        return done(new Error(`token '${token}' not found`));
      }
  
      return tokenObj.destroy(done);
    });
  }
docs: Destroy a token. Username is required as we use it to search through
  tokens.
  
  @param {String} username
  @param {String} token
  @param {Function} done - done(err)