Remove the TODO header in doc builds for index.rst.

The sphinx.ext.todo seem not to be working…

It is supposed to work, I believe, by putting the following into a
docstring:

    class Foo(object):
        """A foo.

        .. todo:: Do something about bar.
        """

And then the `.. todolist::` directive in Sphinx will find all such
`.. todo::` directives in your code, and put them in a list (the way I
was doing it, these would be on the index.html page) with links to where
the `.. todo::` is in the source code.

However, this seems to *only* work in docstrings proper in Python, the
following do *not* get picked up by the `.. todolist::` Sphinx
directive:

    * Normal Python inline comments, i.e.:
        # .. todo:: There is this thing we should do.

    * Sphinx attribute inline comments, i.e.:
        #: .. todo:: There is also another thing we should do.

Because all of the TODOs in BridgeDB are currently within normal Python
inline comments, the `.. todolist::` on the index page doesn't include
anything, and thus there is an awkward looking, giant, blank header with
nothing underneath it. Thus, I'm taking out the header, but leaving in
the `.. todolist::` directive (so that we *can* use `.. todo::` in a
proper docstring in the future and it will be automatically included in
the documentation here).