Do something about FontManager.default_size

[jwiggins]

See discussion on #707.

The default_size attribute comes into play if you call FontProperties.get_size_in_points or pass a FontProperties with size equal to a string from the font_scalings dictionary. However, I'm pretty sure no code actually does this.

In both of the above cases, you will fail unless you explicitly set FontManager.default_size to a value which can be multiplied with numbers (ie: not None). _load_from_cache_or_rebuild is explicitly resetting default_size to None every time a FontManager is loaded from cache.

[jwiggins]

I can't find a single use of default_size when searching all of GitHub. I know that's just a subset of the possible users of this code, but given that using font_scalings was broken for over a decade I think it's probably save to remove it.