It's an immobilizer. Back in the late 1990's and early 2000's a lot of people had them installed. Basically, it routes the starter wire through a relay for starter kill, but instead of using the output of a car alarm to click the relay, it uses a flat "key" that has to be inserted in order to allow the starter signal to pass through the relay.

If you want to remove it, reconnect the black/white wire to itself on the main harness, and remove the rest of it.