It's just an RF suppression choke. It provides a high impedance to RF signals (looking at the image, I'd estimate from around 50MHz upwards).
To cut a long story short, it isn't necessary for the circuit to work. TL1 reduces emissions from the circuit above a certain frequency, and provides some immunity from RF radiated into the connecting cable from an external source. From a design point of view, it seems strange why you'd include this into one of the external lines, but not the others.
I'd suggest not fitting TL1. But it's usually good design practice to fit a 1nF (or whatever) ceramic capacitor between each external signal line and ground, close to where the cable enters the PCB.