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/media.rs

@@ -37,18 +37,15 @@ impl Media {
 #[derive(Debug)]
 pub enum Media {
     Illustration {
-        title: Option<String>,
         url: String,
         thumbnail_url: Option<String>,
         fallback_url: Option<String>,
     },
     Video {
-        title: Option<String>,
         url: String,
         thumbnail_url: String,
     },
     Animated {
-        title: Option<String>,
         url: String,
         thumbnail_url: String,
     },