Issue 2408: remove the _types module

Issue 2408: remove the _types module
It was only used as a helper in types.py to access types (GetSetDescriptorType and MemberDescriptorType),
when they can easily be obtained with python code.
These expressions even work with Jython.

I don't know what the future of the types module is; (cf. discussion in http://bugs.python.org/issue1605 )
at least this change makes it simpler.