The recent changes to fix modalbox focus management with multiple

modalboxes on screen had a couple of bugs with modalbox instance

management that led to over-aggressive focus control.
The code to remove a modalbox from the list was actually doing two

things:


Removing all modalboxes after the modalbox's index, not just the one

   we wanted to remove.


Taking the discarded modalboxes and setting that as the new array.


This resulted in the latest modalbox (and any after it) being the new

set of modalboxes, and ensuring focus on elements outside of that list

would always be stolen and returned to the now-defunct modalboxes. That

issue manifested in Review Board, where attempting to edit a field after

closing the Review Dialog would fail.
We now splice the entry out correctly, and discard the list of removed

modalboxes.

Tested the repro case:

Click Edit Review
Click Cancel
Edit a field.

This worked as expected. I added console logs for the modalBoxes array

during this process and verified we had the correct state after closing

modalboxes.
I also verified the above with nested modalboxes (Edit Review -> Delete

confirmation dialog), ensuring state was correct.
Unit tests pass.