Remove user from role.

def roles_remove(user, role):
    """Remove user from role."""
    user, role = _datastore._prepare_role_modify_args(user, role)
    if user is None:
        raise click.UsageError('Cannot find user.')
    if role is None:
        raise click.UsageError('Cannot find role.')
    if _datastore.remove_role_from_user(user, role):
        click.secho('Role "{0}" removed from user "{1}" '
                    'successfully.'.format(role, user), fg='green')
    else:
        raise click.UsageError('Cannot remove role from user.')