Font underline attribute is never used

Kiva fonts have an `underline` attribute, but `underline` is not a property of the underlying fonts that we query through `kiva.fonttools`, nor is it typically handled by the underlying backend code (perhaps with the exception of SVG).

We should either:

- ensure that where a backend doesn't handle drawing of underlines, that the GC draws an appropriate line; or
- remove underline from the kiva `Font` class and associated code and rely on higher-level tools to draw appropriate underlines

There is a 3rd possibility, which is that the underline property should be being used to inform the font query, but I think that typically all TTF fonts can provide underline positions and don't have distinct glyphs for underlined text, so it is not something that contributes to a search.