secondary midi dev can't (easily) get removed

On Windows 7 64-bit:

1. Set two midi output devs, and click "Ok"
2. Set the 2nd midi output dev to "None" and click "Ok"
3. Open the prefs again

Bug: The preferences dialog still shows the second midi output dev. Somehow it doesn't forget ancillary devs until you set all of those devs to "None" and click "Ok"