fix(P5): review P0 — remove duplicate delenv in attestation signature-skip test

Code review (correctness lens) found a P0 in tests/test_attestation_matrix.py
introduced by the P5 fallback-removal pass: the dual-read delenv of
ACDL_ATTESTATION_SIGNING_KEY_ID was replaced with a second (duplicate)
delenv of NOVA_ATTESTATION_SIGNING_KEY_ID, leaving the test misleading
(comment claimed 'both NOVA_* and ACDL_* must be unset' while only NOVA_*
was deleted twice) and the ACDL_* var no longer cleaned. With P5 having
removed the ACDL_* fallback from core/env.py, deleting NOVA_* alone is the
correct and sufficient precondition for the skip; this commit drops the
duplicate line and updates the comment to match the NOVA-only contract.

---ci---
project: acdl
phase: 5
milestone: v1.15
status: verify
lessons:
  - P0 fix applied: duplicate monkeypatch.delenv('NOVA_ATTESTATION_SIGNING_KEY_ID') in test_signature_skip_when_key_unset left the test misleading and the ACDL_* var uncleaned; collapsed to a single NOVA_* delenv consistent with the P5 NOVA-only core/env.py.
---/ci---

tests/test_attestation_matrix.py

@@ -102,9 +102,10 @@ def test_dr_blocks_on_missing_dr_drill():
 
 def test_signature_skip_when_key_unset(monkeypatch, capsys):
     """D-089: signature verification is skipped when the signing key is unset."""
-    # P2: dual-read — both NOVA_* and ACDL_* must be unset for the skip.
+    # P5 (REQ-164): dual-read fallback removed — NOVA_* only. Deleting
+    # NOVA_ATTESTATION_SIGNING_KEY_ID is sufficient for the skip (the
+    # ACDL_* fallback no longer exists in core/env.py).
     monkeypatch.delenv("NOVA_ATTESTATION_SIGNING_KEY_ID", raising=False)
-    monkeypatch.delenv("ACDL_ATTESTATION_SIGNING_KEY_ID", raising=False)
     artifact = {"timestamp": datetime.datetime.now(datetime.timezone.utc).isoformat(),
                 "type": "x", "payload": {}, "signature": "sig"}
     assert _verify_signature(artifact) is True