The current default of having a dedicated option for the maintainer is a
bit annoying:
- If someone is using a cookiecutter template, they are most likely
starting a new project, which means they are both the author and the
maintainer. In this case, only author needs to be set.
- The current default of null doesn't entirely work, and you're left
with things like `__maintainer__ = 'None <None>'` in a few places.
Having to remove these by hand is a little tedious.
To keep with the minimalism, this commit just removes the option to set
maintainer_name and maintainer_email.
