[identity] Remove social proof from devices table

Summary:
Part 2 of ENG-6480.
Since the social proof is stored in the users table, we don't need to store it in the devices table.

Depends on D10764

Test Plan: Repeated test plan from D10764. Verified that the social proof is still retrieved correctly, especially in GetKeyserverKeys RPC (for which I changed the code).

Reviewers: varun, will, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10765