[lib] delete CallSingleKeyserverEndpointResultInfoInterface

Summary:
ENG-8424.

Later in the stack, we need to return SendMessagePayload (which has +interface: CallSingleKeyserverEndpointResultInfoInterface) as a result of DM Ops. which is encrypting and queueing messages on Tunnelbroker, not keyserver call.

Alternatives:

1. Use tunnelbroker instead of none - I think this is even worse for readability because the type is associated with Keyserver.
2. Remove interface from SendMessagePayload - this seems the best and I think it's safe because I wasn't able to find any usage of interface.

I am not familiar with why we need this at all, so curious about the reviewers' perspective.

Depends on D13319

Test Plan: Flow

Reviewers: tomek, inka, ashoat

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13321