feat: remove frontend code to delete spcp cookie

[tshuli]

Problem

• Hotfix was introduced in feat: client to delete spcp cookie #2328 to temporarily allow client to delete sp/cp cookie for backward compatibility
• As affected cookies have all expired, this PR removes the client code to delete the cookie
• Closes TODO: Remove frontend code to delete spcp cookie #2329

Breaking Changes

• Yes - this PR contains breaking changes
  • Details ...
• No - this PR is backwards compatible (all affected cookies have expired)

New environment variables:

• env var : env var details

OLD_SPCP_COOKIE_DOMAIN env var deleted

Tests

• Create sp form and log in
  • Check that form can be submitted
  • Check that user can log out and jwt cookie is deleted thereafter
  • Log in again. Check that submission and log out works
• Repeat above with 'remember me' enabled
• Repeat above with CP form
• Repeat above with MyInfo form
• Repeat above with SGID form

[karrui]

lgtm with comment

[timotheeg]

LGTM