Remove remaining `__cmp__` in `comtypes`

[junkmd]

In Python3,

The cmp() function should be treated as gone, and the __cmp__() special method is no longer supported. https://docs.python.org/3/whatsnew/3.0.html#ordering-comparisons

However, they are still contained in comtypes.

https://github.com/enthought/comtypes/blob/afc6488d3386771f69ab4a3e305099e16c34f4d7/comtypes/client/lazybind.py#L129-L132

https://github.com/enthought/comtypes/blob/afc6488d3386771f69ab4a3e305099e16c34f4d7/comtypes/GUID.py#L45-L48

https://github.com/enthought/comtypes/blob/afc6488d3386771f69ab4a3e305099e16c34f4d7/comtypes/__init__.py#L592-L605

Remove these dead code and modernize the codebase.