fix bug caused by old helper to support doctest and zope.testing.doctest

These are the source of these types of failures:

  File "testfixtures/tests/test_docs.py", line 7, in <module>
    from manuel import doctest, capture
  File "manuel/doctest.py", line 5, in <module>
    doctest = manuel.absolute_import('doctest')
  File "manuel/__init__.py", line 29, in absolute_import
    if module is None or not hasattr(module, '__file__'):
  File "py/_apipkg.py", line 169, in __getattribute__
    return getattr(getmod(), name)
  File "py/_apipkg.py", line 154, in getmod
    x = importobj(modpath, None)
  File "py/_apipkg.py", line 47, in importobj
    module = __import__(modpath, None, None, ['__doc__'])
ImportError: No module named 'pytest'

The code isn't needed anymore.