[7936] Performance profiles sign off in register guidance update

[tomtrentham]

Context

Feedback from testing has highlighted the need for some guidance to be updated.

Trello card (comment)

Changes proposed in this pull request

• Update guidance for inside performance profile period according to the Word doc
• Tidy up code that's now not used
• Reformat ERB in 'outside period' page

Guidance to review

• Does it look right?
• Do dates display correctly?

Important business

• Does this PR introduce any PII fields that need to be overwritten or deleted in db/scripts/sanitise.sql?
• Does this PR change the database schema? If so, have you updated the config/analytics.yml file and considered whether you need to send 'import_entity' events?
• Do we need to send any updates to DQT as part of the work in this PR?
• Does this PR need an ADR?

NB: Please notify the #twd_data_insights team and ask for a review if new fields are being added to analytics.yml

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.16%. Comparing base (d661e02) to head (8e7c3de).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4907      +/-   ##
==========================================
+ Coverage   96.06%   96.16%   +0.10%     
==========================================
  Files         742      743       +1     
  Lines       16040    16047       +7     
==========================================
+ Hits        15409    15432      +23     
+ Misses        631      615      -16     

Flag	Coverage Δ
unittests	96.16% <100.00%> (+0.10%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud