The second o2 sensor is one part of the whole OBD-2 sensor package.
It directly monitors catalytic converter performance and aids in other aspects of engine performance monitoring.

How you "get rid of it" depends on how you've wired your swap and the emissions laws in your state.

As an example, in California... you have to install all of the emissions control equipment from the later vehicle and run the OEM ECU for the year of your B18C1 motor. Otherwise, you won't pass the BAR certification and you can't register the car.

Moved to Civic / Del Sol / EL.