spec shaking v2: Revert to feature flag for protocol 26

[mootz12]

What problem does your feature solve?

Spec shaking v2 requires developers to use a supported build tool to filter the specs. For now, this should remain opt-in for protocol 26 to prevent unexpected build errors. See #1839 for more details on when a build tool like the CLI will be required.

What would you like to see?

Revert #1812

What alternatives are there?

None

[leighmcculloch]

I think we need to revert these two PRs:

1. Fallback to spec shaking v1 if env var is not set #1831
2. Enable spec shaking v2 by default #1812

[leighmcculloch]

It looks like if we revert #1831 with #1850, then #1812 should be almost a clean revert with just a conflict change in the expand tests to regen.