Revert "Move build logic for the core hydra package into its own default.nix."

This reverts commit 949e5865c60323800f8bd5706842348e3e4437bf. This makes release.nix harder to read/maintain IMHO. There already is a Nix expression for Hydra in Nixpkgs that can be used for this purpose.

Created by  Eelco Dolstra  on April 12, 2017
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