Implement RFC044: Accommodate iSHARE Participant Registry without distributed ledger

Implementation of the documentation changes for RFC044 in the OpenAPI definitions.

The change itself, its rationale and the CAB decision are described in the main RFC issue: rfc#2

Target release: 4.0

Changes in this repository:

• Agreement schema, verificationHash: remove the reference to the ledger. No interface change.

The branch for this issue is merged into 4.0-preview once the CAB has approved RFC044.