Can polyfill references be removed?

[clintfred]

When working on #97 I came across several references to polyfill as a fallback for cryptographic operations. I don't think these are probably needed anymore.

eg:

    return NativeAes.decryptDeviceAndSigningKeys(devicePrivateKey, signingPrivateKey, symmetricKey, iv).handleWith(() =>
        PolyfillAes.decryptDeviceAndSigningKeys(devicePrivateKey, signingPrivateKey, symmetricKey, iv)
    )

Maybe everything in the PolyFilAes file can be removed? Maybe sjcl can be removed?

[skeet70]

https://caniuse.com/mdn-api_crypto_subtle

We should be able to remove the whole file. The NativeAes operations all use crypto.subtle which seems well supported at this time.