Don't need to attach a cfrag in the "proof only" test.

tests/test_capsule/test_capsule_operations.py

@@ -1,6 +1,6 @@
 import pytest
 
-from umbral import pre, keys
+from umbral import pre
 from umbral.curvebn import CurveBN
 from umbral.point import Point
 from umbral.pre import Capsule

tests/test_correctness.py

@@ -21,9 +21,6 @@ def test_correctness_proof_serialization(alices_keys):
     metadata = b"This is an example of metadata for re-encryption request"
 
     cfrag = pre.reencrypt(kfrags[0], capsule, metadata=metadata)
-
-    capsule.attach_cfrag(cfrag)
-
     proof = cfrag.proof
     proof_bytes = proof.to_bytes()