Yep, that's the HH cable.
You can disconnect it as Josh said, expect your clutch pedal to feel really flimsy though.
There used to be a company affiliated with
CCR that rebuilt HH valves, I think they were called RooRepair or something, but I can't find it anywhere now, so I guess they stopped, which was a shame because the prices were very reasonable.
If you want to do away with the whole thing altogether, then you can get a brake master cylinder off an automatic car and redo the brake lines accordingly.