How to treat un-used input pins for the Rin/RS232 side on MAX3232?
On those devices the RS232 inputs are already pulled to ground internally via a 5K resistor. As such you do not need to attach anything to the pins.

If you wish, you can tie the unused pin to ground. But remember, if you decide to add the handshake functionality later, the choice to short it to ground now can be a real pain to rework. For that reason, I would actually change your design to add a pull-down resistor to the unused T2N pin which can be removed later if need be.