refactor(x-media): drop Media's write-only title field

Every adapter set title (mostly None) on all three Media variants and
nothing ever read it: the bot's CachedMedia carries kind/file_id/url, and the
one read was misskey's own test. Removing it also drops misskey's
DriveFile.name, which existed only to feed it. Fetched::title (the post's
own headline, which captions do use) is untouched.

crates/x-media/src/site/pixiv/api.rs

@@ -149,7 +149,6 @@ impl PixivAPI {
             match self.ugoira_video(illust_id).await {
                 Ok(Some((mp4_path, _keep_alive))) => {
                     illustration.media.push(Media::Video {
-                        title: None,
                         url: mp4_path,
                         thumbnail_url: model.image_urls.medium.clone(),
                     });

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

@@ -235,7 +235,6 @@ impl Illustration {
                     .original
                     .clone()
                     .map(|original| Media::Illustration {
-                        title: None,
                         url: original,
                         thumbnail_url: Some(page.image_urls.medium.clone()),
                         fallback_url: Some(page.image_urls.large.clone()),
@@ -248,7 +247,6 @@ impl Illustration {
             .or(model.image_urls.original.clone())
         {
             media.push(Media::Illustration {
-                title: None,
                 url: original,
                 thumbnail_url: Some(model.image_urls.medium.clone()),
                 fallback_url: Some(model.image_urls.large.clone()),