Update classical_shadows.ipynb

Author: PeilinZHENG

docs/source/tutorials/classical_shadows.ipynb

@@ -466,8 +466,7 @@
    "source": [
     "bz = 1000\n",
     "\n",
-    "exact, res = [], []\n",
-    "exact_expec = c.expectation_ps(ps=ps[0])\n",
+    "exact = []\n",
     "for ob in ps:\n",
     "    exact.append(tc.backend.real(c.expectation_ps(ps=ob)))\n",
     "exact = np.asarray(exact)[:, None]\n",