Don't allow configurable key overrides

[martinthomson]

Allowing sites to selectively override some keys and not others is highly likely to create web compatibility hazards. Different browsers reserve different key combinations for their own use and inconsistent application of this API is likely to create inconsistent experiences.

Limiting this to just those keys the browser would ordinarily reserve for its own purposes is far easier to reason about and communicate. If system keys can be reserved (a separate issue), then the same applies there.