remove invited_user from session entirely

now that we no longer set it since
https://github.com/alphagov/notifications-admin/pull/3841 was merged, we
don't need to remove it either. And we can remove checks that expect it
when cleaning up the session. And the unit tests that make sure we
ignore it if it's in the session.

So long, session['invited_user'] and session['invited_org_user']!

tests/app/main/views/test_accept_invite.py

@@ -178,7 +178,7 @@ def test_accepting_invite_removes_invite_from_session(
     assert normalize_spaces(page.select_one('h1').text) == landing_page_title
 
     with client_request.session_transaction() as session:
-        assert 'invited_user' not in session
+        assert 'invited_user_id' not in session
 
 
 def test_existing_user_of_service_get_redirected_to_signin(
@@ -416,7 +416,6 @@ def test_new_user_accept_invite_completes_new_registration_redirects_to_verify(
     with client.session_transaction() as session:
         assert response.status_code == 302
         assert response.location == expected_redirect_location
-        assert 'invited_user' not in session
         assert session.get('invited_user_id') == USER_ONE_ID
 
     data = {'service': sample_invite['service'],