refactor: remove unused modules

[doc-han]

Description

This PR removes legacy auth-provider modules that became dead code once the new SSO handler architecture landed.

Closes #4723

Validation steps

1. mix compile --warnings-as-errors (no warnings about missing/unused modules).
2. mix test test/lightning_web/live/credential_live_test.exs (credential OAuth tests pass against the updated assertions).
3. mix credo --strict --all and mix dialyzer.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.4%. Comparing base (6883427) to head (39d763a).

Additional details and impacted files

@@                    Coverage Diff                     @@
##           4621-full-sso-experience   #4825     +/-   ##
==========================================================
+ Coverage                      90.2%   90.4%   +0.2%     
==========================================================
  Files                           447     444      -3     
  Lines                         22735   22656     -79     
==========================================================
- Hits                          20507   20486     -21     
+ Misses                         2228    2170     -58     

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[github-actions]

Now I have enough context to produce the security review. The PR's substantive new content is SSO/account-level changes (the workflow_channel.ex diff in my view is reversed recent main commits not yet in the branch — they aren't introduced by this PR).

Security Review ✅

• S0 (project scoping): N/A — all new code is account/auth-layer (SSO identity linking, signup confirmation, new GitHub/Google handlers in lib/lightning_web/controllers/oidc_controller.ex:50-313, lib/lightning/accounts.ex:175-296); no project-scoped resources (dataclips, runs, workflows, etc.) are queried or mutated.
• S1 (authorization): PASS — link/2 gates on conn.assigns.current_user before stashing intent (oidc_controller.ex:38); unlink-identity operates on socket.assigns.user derived from @current_user in the profile LiveView (identities_component.ex:19, profile_live/components.ex:67-70) which already sits behind require_authenticated_user; signup confirmation uses session-stashed state (@pending_signup_session_key), never trusting client-supplied user IDs; the email-collision branch in handle_sso_login refuses to auto-link an unverified provider identity to an existing account (oidc_controller.ex:226-234).
• S2 (audit trail): N/A — user-identity linking is an account-level action, not project/instance configuration, and matches no existing audit module (credentials, projects, workflows, webhook_auth_method, oauth_client, version_control, dataclip, export).

[stuartc]

Nice dude, all good from my side, we do need to drop the dead SALESFORCE_CLIENT_* lines from .env.example and Salesforce section on DEPLOYMENT.md, up to you if you want to do it in this PR or not. Let me know if/when you want me to merge this.