LG-10657 move pii from doc out of flow session STEP 1

[soniaconnolly]

🎫 Ticket

LG-10657

🛠 Summary of changes

Start the process of moving :pii_from_doc into idv_session. Incidentally clean up some controller spec flow_session values that are no longer used.

Plan:

1. Align on :pii_from_doc as flow_session key, replacing 'pii_from_doc' where needed (this PR)
2. Make as many reads as possible go through pii_from_doc method in IdvStepConcern (this PR)
3. Move ssn out of pii_from_doc into idv_session, possibly into applicant, or its own key (two separate PRs, LG-10886)
4. Copy pii_from_doc into idv_session (are there session size issues with this?) (future PR)
5. Remove flow_session[:pii_from_doc] (future PR)

📜 Testing Plan

Provide a checklist of steps to confirm the changes.

• Create account, go through IdV
• On VerifyInfo step, edit address and SSN.
• Expect profile to be created correctly.

[kbighorse]

LGTM 👍🏾

[jmax-gsa]

Nice cleanup.