hive-priv: drop the descriptor-taking snapshot send and its fd-receiving loop

hive-priv-sock/src/lib.rs

@@ -645,34 +645,6 @@ pub enum PrivRequest {
         dest_file_name: String,
     },
 
-    /// Stream a previously-created read-only snapshot into a file
-    /// descriptor the caller passes alongside this request (`SCM_RIGHTS`
-    /// ancillary data on the same socket): `btrfs send [-p <parent>]
-    /// <snapshot> >&<passed fd>`.
-    ///
-    /// The caller connects to the destination, writes any header itself,
-    /// and hands the **connected socket** over — so hive-priv
-    /// never learns an address, a protocol, or that a network is
-    /// involved, and nobody sits in the data path once the send starts
-    /// (which is what makes a multi-gigabyte transfer survive a
-    /// hive-c0re restart). hive-c0re sends no such request today.
-    ///
-    /// Exactly one descriptor must accompany this request. hive-priv
-    /// rejects the request if none arrived, if more than one did, or if a
-    /// descriptor arrives alongside any *other* operation — no guessing
-    /// when the caller didn't say. Requires root.
-    SendAgentSnapshotToFd {
-        /// Logical agent name (validated by `validate_agent_name`).
-        agent_name: String,
-        /// Snapshot label to send, same validation as `SnapshotAgentSubvolume`.
-        snapshot_name: String,
-        /// Optional parent snapshot label for an incremental
-        /// (`btrfs send -p`) send — must be an older read-only snapshot of
-        /// the same agent, still present on disk. `None` sends the full
-        /// snapshot.
-        parent_snapshot_name: Option<String>,
-    },
-
     /// Legacy: an older hive-c0re still sends this. hive-priv only unlinks
     /// `/etc/tmpfiles.d/hyperhive-agents.conf` and returns `Ok`; the `agents`
     /// payload that request carries is ignored on decode. Remove once no