code: def unstage_signature(vcs, signature):
      
      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)
docs: Remove `signature` from the list of staged signatures
  
      Args:
          vcs (easyci.vcs.base.Vcs)
          signature (basestring)
  
      Raises:
          NotStagedError