Investigate `useCategories` option

[jonasfj]

It appears to me that useCategories defaults to true, and that nobody reads it.

    DartdocOptionArgOnly<bool>('useCategories', true, resourceProvider,
        help: 'Display categories in the sidebar of packages'),

   bool get useCategories => optionSet['useCategories'].valueAt(context);

I certainly can't find any references of it in the templates or repository at large.

Perhaps, we should:

• Allow the option to be passed, but ignore it.
• Print a warning that it is deprecated when the option is passed.
• Remove useCategories => optionSet['useCategories'].valueAt(context);

[srawlins]

Ha, I'm all for deprecating then removing it.