Normal install for GM ASCM cars

Can we make the ASCM GM cars a completely normal install? That means a single harness, and we remove the OBD-II harness and the 3rd party L&P harness.

I haven't looked into this at all yet, so not sure it's possible. If it's not, let's close this.

It's fine to lose functionality, such as longitudinal control. A normal install will get the ASCM cars back on https://comma.ai/vehicles.

In order to claim the bounty:
* post the harness diagram
* must work for the currently supported Chevy Volts
* get a PR merged to move the GM ASCM cars to this new harness

Start by determining whether the ASCM GM cars can use a single harness without the OBD-II and third-party L&P harnesses, while supporting the currently supported Chevy Volts. Document the resulting harness diagram, then identify the vehicle configuration entry point and make a PR that moves the GM ASCM cars to the new harness; done means the PR is merged and the cars return to the supported vehicle list.

Written by the indexing model from the issue text.