-XJavaScriptFFI does nothing

The extension JavaScriptFFI has not been doing anything since it was added in b372e8ea.

It should either control foreign import javascript or be deprecated. (For example, the proposed winapi calling convention https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0277-winapi-callconv.rst#proposed-change does not have a flag.)

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