[services][identity] update get_pake_registration method to use helper

Summary: previously introduced a helper function that gets an item from the users table in DDB with the provided user_id. we should use that helper function here.

Test Plan: built and ran the identity service successfully and called the login RPC to check that it works as expected

Reviewers: tomek, bartek, jon

Reviewed By: tomek, bartek, jon

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5573