Remove `signature` from the list of staged signatures

    Args:
        vcs (easyci.vcs.base.Vcs)
        signature (basestring)

    Raises:
        NotStagedError

def unstage_signature(vcs, signature):
    """Remove `signature` from the list of staged signatures

    Args:
        vcs (easyci.vcs.base.Vcs)
        signature (basestring)

    Raises:
        NotStagedError
    """
    evidence_path = _get_staged_history_path(vcs)
    staged = get_staged_signatures(vcs)
    if signature not in staged:
        raise NotStagedError
    staged.remove(signature)
    string = '\n'.join(staged)
    with open(evidence_path, 'w') as f:
        f.write(string)