VPR-214 refactor(web): drop unused session registration

[rlorenzo]

• Nothing reads HttpContext.Session or TempData, so the session store
  only existed to be configured
• Blue/green slot flips require no per-user state in process

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[codecov-commenter]

Bundle Report

Bundle size has no change ✅

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 45.37%. Comparing base (4b9da22) to head (1101096).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #342   +/-   ##
=======================================
  Coverage   45.37%   45.37%           
=======================================
  Files         948      948           
  Lines       49532    49532           
  Branches     6700     6700           
=======================================
  Hits        22477    22477           
  Misses      26089    26089           
  Partials      966      966           

Flag	Coverage Δ
backend	42.34% <ø> (ø)
frontend	64.72% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
web/Classes/HealthChecks/HealthCheckExtensions.cs	0.00% <ø> (ø)