Remove redundant footnote (#181)

It was accidentally added in cc798377.

Doc/tutorial/datastructures.rst

@@ -107,7 +107,7 @@ An example that uses most of the list methods::
 
 You might have noticed that methods like ``insert``, ``remove`` or ``sort`` that
 only modify the list have no return value printed -- they return the default
-``None``. [1]_  This is a design principle for all mutable data structures in
+``None``.  This is a design principle for all mutable data structures in
 Python.