remove unused Content::new method

src/atrule/mixin.rs

@@ -29,12 +29,3 @@ pub(crate) struct Content {
     pub content: Option<Vec<Token>>,
     pub content_args: Option<FuncArgs>,
 }
-
-impl Content {
-    pub const fn new() -> Self {
-        Self {
-            content: None,
-            content_args: None,
-        }
-    }
-}