code: public void removeSigner(String email) throws HelloSignException {
          if (email == null) {
              throw new HelloSignException("Cannot remove null signer");
          }
          for (int i = 0; i < signers.size(); i++) {
              if (email.equalsIgnoreCase(signers.get(i).getEmail())) {
                  signers.remove(i);
              }
          }
      }
docs: Removes the signer from the list. If that user does not exist, this will
  throw a HelloSignException.
  
  @param email String
  @throws HelloSignException thrown if there is a problem removing the
  signer.