Remove redundant symmetric-kappa call from trimming fix cell

The tutorial cell that demonstrates the asymmetric window fix was
re-running StackedDiD(kappa_pre=3, kappa_post=3), which re-emitted
the trimming warning that was already shown and explained in the
preceding cells. Remove it so the "fix" cell is warning-free.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: igerber

docs/tutorials/13_stacked_did.ipynb

@@ -244,18 +244,7 @@
    "execution_count": null,
    "metadata": {},
    "outputs": [],
-   "source": [
-    "# Symmetric kappa=3 trims cohort 7\n",
-    "r_sym = StackedDiD(kappa_pre=3, kappa_post=3).fit(\n",
-    "    data, outcome='outcome', unit='unit', time='period', first_treat='first_treat')\n",
-    "print(f\"Symmetric [-3, 3]:  groups={r_sym.groups}, trimmed={r_sym.trimmed_groups}\")\n",
-    "\n",
-    "# Asymmetric window recovers cohort 7 — no warning this time\n",
-    "r_asym = StackedDiD(kappa_pre=3, kappa_post=2).fit(\n",
-    "    data, outcome='outcome', unit='unit', time='period', first_treat='first_treat')\n",
-    "print(f\"Asymmetric [-3, 2]: groups={r_asym.groups}, trimmed={r_asym.trimmed_groups}\")\n",
-    "print(f\"\\nAll 3 cohorts included. ATT={r_asym.overall_att:.3f} (SE={r_asym.overall_se:.3f})\")"
-   ]
+   "source": "# Asymmetric window recovers cohort 7 — no warning this time\nr_asym = StackedDiD(kappa_pre=3, kappa_post=2).fit(\n    data, outcome='outcome', unit='unit', time='period', first_treat='first_treat')\nprint(f\"Asymmetric [-3, 2]: groups={r_asym.groups}, trimmed={r_asym.trimmed_groups}\")\nprint(f\"\\nAll 3 cohorts included. ATT={r_asym.overall_att:.3f} (SE={r_asym.overall_se:.3f})\")"
   },
   {
    "cell_type": "markdown",
@@ -461,4 +450,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 4
-}
+}