This code was added in 9dc5dfb975
and 704050da23 because the browser-
native checkbox was `display: none`, breaking native keyboard
accessibility.
The native checkbox is now merely `appearance: none`, which does
not turn off [behavior semantics], so JavaScript to
reimplement it isn't needed any more.
[behavior semantics]: https://w3c.github.io/csswg-drafts/css-ui/#appearance-semantics