[identity] Stop adding devices to users table

Summary:
Part of ENG-5842.

Removed code that adds devices to the users table. This is no longer needed since we add devices to the devices table now.

Test Plan:

• Identity integration tests
• Manually registered a device and used awslocal dynamodb scan --table-name identity-users to verify that the device was not added to the users table.

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10698