Remove outdated performance comment in markdown

ipynb/palsum.ipynb

@@ -220,7 +220,7 @@
    "id": "f4db4656-6146-4bae-9235-eae95943dd5a",
    "metadata": {},
    "source": [
-    "It takes about a second to run the test suite on cases up to *N* = 44 (for which there are 4,011,032 valid palindromes)."
+    ""
    ]
   },
   {