refactor(site): drop RenderData's url field, source_url already carries it

Every adapter built RenderData { url } from the exact value it then moved into Fetched.source_url — a copy of the same canonical URL held in two places, feeding one placeholder. caption_with reads {url} from self.source_url now and the field is gone from the struct (bsky's and twitter's halves of this change landed with their caption commits; their render builders no longer set it either).

crates/x-media/src/site/misskey/interface.rs

@@ -133,7 +133,6 @@ impl From<model::Note> for Fetched {
             sensitive,
             site_id: "misskey",
             render_data: Some(RenderData {
-                url,
                 author: encode_text(&author).into_owned(),
                 author_url: author_url.clone(),
                 title: String::new(),