Remove maintainer_name and maintainer_email

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.

{{cookiecutter.package_name}}/README.rst

@@ -34,6 +34,3 @@ Authors
 -------
 
 `{{ cookiecutter.package_name }}` was written by `{{ cookiecutter.author_name }} <{{ cookiecutter.author_email }}>`_.
-{% if cookiecutter.maintainer_name -%}
-`{{ cookiecutter.package_name }}` was written by `{{ cookiecutter.maintainer_name }} <{{ cookiecutter.maintainer_email }}>`_.
-{%- endif %}