docs: name the Configuration tab as it is rendered, drop an unused fixture

Round 6's two remaining nits. The operator note said "Configuration → Email"
but the group renders as "Email / SMTP" in both locales, and
test_an_admin_can_activate_a_pre_approved_registrant requested make_user without
using it — a leftover from the reverted rounds that made the test look like it
seeded an admin when it did not.

backend/tests/test_auth_session.py

@@ -209,7 +209,7 @@ def test_registration_with_an_already_pending_request_persists_the_account(
     assert notified == ["pia@example.com"]
 
 
-def test_an_admin_can_activate_a_pre_approved_registrant(db, make_user, monkeypatch):
+def test_an_admin_can_activate_a_pre_approved_registrant(db, monkeypatch):
     # The recovery path for the account above, pinned so it cannot quietly break: re-entering the
     # address in the Access-requests whitelist box (POST /api/admin/invites) runs
     # _activate_user_by_email, which is also what keeps the SEC-10 preset-password guard in play.