Remove any `c:tx[c:rich]` child, or do nothing if not present.

def remove_tx_rich(self):
        """
        Remove any `c:tx[c:rich]` child, or do nothing if not present.
        """
        matches = self.xpath('c:tx[c:rich]')
        if not matches:
            return
        tx = matches[0]
        self.remove(tx)