--extra-python removal

This removes the --extra-python configure option.

We now support Python 3.4 for everything and no longer test this in autobuild, so this should be removed.

Multiple Python versions can be tested and built for multiple pythons by running the full build multiple times.

This removes an intrusive change in waf that is no longer required.

(To be clear, no change to minimum python versions is made in this patch, that will be handled separately.