style: Move some parsing-only attributes to use #[parse(..)] instead of #[css(..)].

I need to admit I'm ambivalent about this one :).

Bug: 1466609
Reviewed-by: xidorn
MozReview-Commit-ID: F1jlfnQKXwo

components/style_derive/to_css.rs

@@ -234,8 +234,6 @@ pub struct CssVariantAttrs {
     pub comma: bool,
     pub dimension: bool,
     pub keyword: Option<String>,
-    pub aliases: Option<String>,
-    pub parse_condition: Option<Path>,
     pub skip: bool,
 }