I'm pretty certain that's just a feed to the MAP/BARO solenoid, which directs the input to the pressure sensor from either the manifold or the ambient air depending on whatever parameters determine which is needed. The line in question just goes to the airbox ahead of the filter to supply the BARO input, which is essentially just getting a reading equivalent to anywhere else, you can just leave the line disconnected and it shouldn't know any different.

Brent