Deletes the user and all related metadata with the given username.

def DeleteGRRUser(self, username, cursor=None):
    """Deletes the user and all related metadata with the given username."""
    cursor.execute("DELETE FROM grr_users WHERE username_hash = %s",
                   (mysql_utils.Hash(username),))

    if cursor.rowcount == 0:
      raise db.UnknownGRRUserError(username)