Issue #3485: remove misleading comment

Lib/posixpath.py

@@ -48,7 +48,6 @@ def _get_sep(path):
 
 def normcase(s):
     """Normalize case of pathname.  Has no effect under Posix"""
-    # TODO: on Mac OS X, this should really return s.lower().
     if not isinstance(s, (bytes, str)):
         raise TypeError("normcase() argument must be str or bytes, "
                         "not '{}'".format(s.__class__.__name__))