cleanup: remove duplexify dependency

[talentlessguy]

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removes unused duplexify dependency, that was removed in #415

all tests pass, nothing changed

[talentlessguy]

pardon me, apparently it was still used, will commit a removal of it in favor of stream.Transform (or stream.Duplex)

[talentlessguy]

I'm gonna close this PR, because duplexify is not easy to replace with stream.Duplex since the first allows dynamically setting readable and writable, which is not possible with Node streams