Always use a separate database for blobs (#4892)

* Always use a separate blobs DB

* Add + update tests

beacon_node/store/src/metadata.rs

@@ -135,7 +135,7 @@ pub struct BlobInfo {
     /// If the `oldest_blob_slot` is `None` then this means that the Deneb fork epoch is not yet
     /// known.
     pub oldest_blob_slot: Option<Slot>,
-    /// A separate blobs database is in use.
+    /// A separate blobs database is in use (deprecated, always `true`).
     pub blobs_db: bool,
 }