Removed incorrect statement from docstring

The docstring of FilterExpression said that it shouldn't be
instantiated from anywhere but the get_filters_from_token
helper function.

However, that helper function was deleted in commit
3ede006fc98f7e96ae9fb997872f78635576d5f8 and FilterExpression
is instantiated from inside the compile_filter help function.

django/template/base.py

@@ -545,9 +545,6 @@ class FilterExpression(object):
         2
         >>> fe.var
         <Variable: 'variable'>
-
-    This class should never be instantiated outside of the
-    get_filters_from_token helper function.
     """
     def __init__(self, token, parser):
         self.token = token