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/misskey/model.rs

@@ -30,6 +30,4 @@ pub(crate) struct DriveFile {
     pub(crate) thumbnail_url: Option<String>,
     #[serde(default, rename = "isSensitive")]
     pub(crate) is_sensitive: bool,
-    #[serde(default)]
-    pub(crate) name: Option<String>,
 }