Remove non Python3 compatible files

[lesteve]

Quoting @pv in #70:

I believe these file are not widely used, so porting them to Python 3 may not be worthwhile. I don't think deleting them will cause a large inconvenience for anyone.

Fixes #70.
Closes #8. Closes #35.

[lesteve]

I quickly looked for numpydoc traitsdoc via the github web UI and could not find any usage outside that was not test_traitsdoc.py. Same thing for plot_directive. I am happy to search more systematically via the github API if that's deemed necessary.

[stefanv]

/cc @rkern @cournape

[rkern]

I think all uses of traitsdoc have been vendorized, so it is not necessary to distribute it with numpydoc.

[stefanv]

Thanks, Robert. Merging.