Remove the complicated nix-support/*specBench stuff

[Chris Warburton]

These used to be standalone commands, which looped through many samples,
timed themselves, etc.

Since we're using asv for this now, this extra cruft can be removed. We
can probably do a LOT of consolidation, deduping and garbage collection.

Note that we should only do this once we've got QuickSpec, HashSpec and
MLSpec benchmarks for asv, for both Isabelle theories and TIP samples.