Scrap compatibility junk from scripts/default.nix

[Chris Warburton]

At the top of scripts/default.nix there are some gubbins to reach inside
old versions of te-benchmark and pull out things like the benchmark dir,
the racket environment, etc.

These were needed so that we could interoperate with old versions of
te-benchmarks, which didn't directly expose some of the things we
need. We needed to interoperate with those versions to make sure our own
Isabelle runs are using the same benchmark as any Haskell versions we're
comparing against.

We've recently spotted a problem in the old benchmark version, in that
it was including destructors and their wrapper functions, which will
artificially deflate the precisions (since the destructors themselves
would never have any corresponding theorems in the ground truth set,
despite the (equivalent) destructor functions possibly having some).

The benchmark has been updated, but since those old results are dodgy,
there's little point going to these lengths to support them. We might as
ensure that the fixed benchmarks expose everything we need, then re-run
the Haskell version with this new benchmark, then this Isabelle version
won't have to mess around to be consistent.