Remove workaround for scalacheck

[ttim]

This PR addresses https://github.com/twitter/summingbird/issues/741 issue with serialization of generated scalacheck lambdas. We can safely remove ArbitraryWorkaround class with newest scalacheck release.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Timur Abishev seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}