This is a performance optimization: Rather than copying these files
into the `prod-static` directory and then deleting them, we just don't
copy them over in the first place.
For styles, it might have once been the case that this did something,
but we've moved them all to being managed by webpack some time ago.
For the js directory, I think it was never useful to copy and then
delete them; these files were always compiled via tools/minify-js,
and the raw JS files weren't needed, anyway.
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