Remove "Toggle keyboard nav" action

[microbit-matt-hillsdon]

@kmcnaught and I are not convinced we need a "Toggle keyboard nav" action and suggest removing it.

We're working towards a scenario where keyboard navigation can be enabled by default and not cause issues for users who don't want to use it.

For user testing we've hidden it from the shortcut docs via a recent commit on #250.

[rachel-fenichel]

I agree: we don't need it.