Update cf/test/test_Field.py

Co-authored-by: Sadie L. Bartholomew <sadie.bartholomew@ncas.ac.uk>

Author: davidhassell

cf/test/test_Field.py

@@ -3462,7 +3462,6 @@ def my_mean(a, axis=None):
                 f.healpix_decrease_refinement_level(level, "mean")
 
         # Can't change refinement level for a 'nuniq' field
-        # TODOHEALPIX
         with self.assertRaises(ValueError):
             self.f13.healpix_decrease_refinement_level(0, "mean")