Drop special-case hack when the initial and the target Python have
the same version. This used the initial Python to generate the
bin/buildout for the target Python. This could cause issues because
the initial and the target Python are unlikely to share the same
sites-packages, and this influences script generation.

The new rebootstrap mechanism also works when the Python versions
are the same, so it can be used in all cases.

Removing this hack also reduces even more the dependency on private
internal APIs of zc.buildout.