Proposal: Eliminate VRC event type

[stevetodd]

Since transferable identifier receipt quadruples can be attached to events and receipts, it is redundant to continue to have a VRC event. I propose that we eliminate the VRC event type and only use RCT. We probably want to wait until after IIW so we don't mess up demos.

[SmithSamuelM]

It would break code to instead of attaching the receipt couple to attach a quadruple instead. Also attaching quadruples is much more verbose than attaching indexed signatures when multisig. So although it may be possible to do it without trans receipt message its much less compact. Better would be to wait until we have a group type that supports a triple plus a group of indexed signatures. This is on a todo list for conjoint replay mode because when multisig signers of non-trans receipt, conjoint replay mode is less compact than disjoint replay mode by virtue of the quadruples (one per signer). So once we have the latter (ie group of triple plus group if indexed signatures) then it would make sense to upgrade the disjoint receipt which would break code to support the new attachment type. And then at such time it might make sense to pare away the functionality by removing the VRC nontrans receipt message. But it would break a lot of code so its not a high priority. Higher is adding triple plus group indexed sigs to conjoint replay mode. But that is also relatively low priority compared to getting witnesses running.

[stevetodd]

IIRC, the group approved this, so I think this issue could be closed. It appears to have already been done in keripy (https://github.com/decentralized-identity/keripy/pull/158).