unprepare method

Implementation

void unprepare() {
  // Some SQLConnections does not support statement [un]preparation,
  // but they have allocated local cursor(s) so let them do cleanup
  // f.e. cancel pending queries and/or free resultset and also do
  // UnRegisterStatement! (original comment kept as-is)
  if (_cursor != null) {
    doUnPrepare();
  }
}