Remove roboto dependency from paper-styles

[gar1t]

The roboto font is a dependency on a hard-coded CDN. Applications can add the dependency and include the font as needed, rather than rely paper-styles, which is used extensively throughout the Polymer components. This is similar to other fonts listed in default styles that might not be available on a system by default, but can be resolved externally at the application level.

This change does not provide backward compatibility or a deprecation period.