matrix: drop the per-hive appservice and its unused token

Every hive's homeserver loaded a per-hive `hyperhive` appservice
registration, and the swarm minted and delivered an as_token for it
through bao. No client presented that token: swarm-controller creates
every `@hive-<hive>` sender and every agent account with the swarm
appservice token (hive_sender.rs, agent_token.rs), and no Rust code read
the per-hive one.

Removed, with everything that existed only to serve it:
- hive-matrix.nix: the `hyperhive` registration, its activation-time
  render, the host bind mount of /var/lib/hyperhive/matrix-appservice,
  its LoadCredential entry, and the internal options
  `deploy.matrix.appserviceTokenFile` / `appserviceRegistrationScript`.
  tuwunel now loads only the swarm registration.
- glue-matrix-bao-token.nix and its `swarm-bao-matrix-token` unit.
- glue-bao-tls.nix: the matrix-token leaf and its defaults.
- swarm-bao.nix: the `swarm-matrix-token-<hive>` roles and policies
  (`swarm-bao-matrix-token-policy`), and the options
  `deploy.bao.matrixTokenClient{Cert,Key}File` and
  `matrixTokenCommonNamePrefix`, now mkRemovedOptionModule.
- swarm-secret-publisher.nix: the per-hive token mint and its state dir.
- swarm.nix: the reserved `<matrixTokenCommonNamePrefix>-<hive>` CN.
- swarm-secret-client: `matrix::appservice_token_path`.

The swarm registration (swarm-matrix-ctl, swarm-controller) is untouched.
Comments and docs that pointed at the removed glue file now point at
glue-queue-agent-credential.nix, which has the same reader shape.

Refs #4428

swarm-matrix-client/src/lib.rs

@@ -500,8 +500,8 @@ mod tests {
     #[test]
     fn the_expected_already_exists_answer_is_recognised_by_its_errcode() {
         // Matched on the spec's code rather than on message text, because this
-        // is the arm a healthy homeserver takes every time: the hive's `@hive-<hive>:`
-        // account is the appservice registration's own sender, created at startup.
+        // is the arm a healthy homeserver takes every time: an account minted
+        // on an earlier pass already exists.
         let json = serde_json::json!({ "errcode": "M_USER_IN_USE", "error": "User ID taken" });
         assert_eq!(errcode(&json), Some("M_USER_IN_USE"));
     }