LibWeb: Simplify handling of font-variation-settings

Since we resolve any relative lengths at compute time there's no need
for the value to be passed around as a `NumberOrCalculated` and we can
just resolve it within `ComputedProperties::font_variation_settings`.

The only place this is used it is used with value_or so there's no need
to return it is an `Optional`.

This is only used for loading fonts (which occurs during style
computation) so there's no need to store it in `ComputedValues`

Libraries/LibWeb/HTML/Canvas/CanvasTextDrawingStyles.cpp

@@ -148,8 +148,7 @@ void CanvasTextDrawingStyles<IncludingClass, CanvasType>::set_font(StringView fo
                 computed_font_style->as_font_style().to_font_slope(),
                 computed_font_weight->as_number().number(),
                 computed_font_width->as_percentage().percentage(),
-                {},
-                length_resolution_context);
+                {});
         },
         [](HTML::WorkerGlobalScope*) -> RefPtr<Gfx::FontCascadeList const> {
             // FIXME: implement computing the font for HTML::WorkerGlobalScope