Remove nunique from not implemented list

Author: sfc-gh-jkew

tests/integ/modin/pivot/test_pivot_table_negative.py

@@ -168,7 +168,6 @@ class Baz:
         (np.any, "np.any"),
         ("size", "'size'"),
         (len, "<built-in function len>"),
-        ("nunique", "'nunique'"),
         ("idxmax", "'idxmax'"),
         ("idxmin", "'idxmin'"),
     ],