[DO NOT MERGE] remove: remove old sidechain design features

[mvadari]

High Level Overview of Change

This PR essentially reverses #1940. It removes all the features related to the old sidechain design.

This PR will not be merged until #2039 is ready.

Context of Change

These are no longer necessary for the new sidechain design.

Type of Change

• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)

While this is technically a breaking change, it's removing support for a feature that never existed in production rippled, so we don't need a major version bump.

Test Plan

CI passes.