refactor: remove unused QuantumCircuit import and stale variable definition

Author: johngeorgea157-stack

mitigation/pec.py

@@ -15,7 +15,7 @@
 """
 
 import numpy as np
-from qiskit import QuantumCircuit, ClassicalRegister
+from qiskit import ClassicalRegister
 from qiskit_aer import AerSimulator
 
 from mitigation.base import Mitigator

mitigation/tests/test_pec.py

@@ -98,7 +98,6 @@ def test_noisy_choi_has_lower_trace(self):
 
     def test_trace_monotonic_with_error(self):
         """Trace should decrease monotonically with error probability."""
-        choi_ideal = compute_choi_matrix_ideal_1q()
         traces = [
             np.trace(compute_choi_matrix_noisy_depolarizing(p)) for p in [0.001, 0.005, 0.01, 0.02]
         ]