swarm CA: retire the host-held root CA, the store holds the only anchor

The secret store's pki root issues every hive's gateway leaf and the
swarm's service names, so the root swarm-ca.service generated on the
host anchors nothing and its key is a secret outside the store.

- swarm-ca.nix is gone: swarm-ca.service, root.pem and root-key.pem,
  and the swarm.ca.{autoConfigure,stateDir,validityDays} options. A
  definition of any of them fails evaluation with a removal message
  (swarm-ca-removed.nix), and a tmpfiles rule deletes /var/lib/swarm-ca
  on every host. singleHostSwarm no longer sets a swarm.* option.
- hive-tls-ca drops the $root slot: trust-bundle.pem holds the store's
  pki root and a hive CA's public ca.pem/ca-previous.pem only.
- hivectl peer-config no longer prints the scp line for the swarm root;
  the store's pki root is the one step.
- The gateway's cert import no longer stands a hive leaf or a throwaway
  in for a missing swarm-services leaf where it Requires= the issuing
  unit: it fails, naming swarm-services-cert.service. The stand-in stays
  where the gateway does not wait (no swarm name, or the store's UI
  only, whose passthrough must start with the store sealed).

Comments and docs that named swarm-ca.nix, swarm.ca.* or the host root
now name the store's pki root.

Refs #4588

nix/host-modules/swarm-peers-removed.nix

@@ -57,11 +57,10 @@ in
       services.hyperhive.swarm.peers.<hive>.caCert is removed and
       ignored, and has no replacement in services.hyperhive.swarm.hives.
 
-      Trust inside a swarm now derives from the swarm root CA
-      (services.hyperhive.swarm.ca — see docs/swarm/ca.md): every hive
-      under it chains to it, so a per-hive CA is dead weight. What this
-      genuinely drops is trusting a hive whose root this swarm does NOT
-      own — another swarm's, or one keeping its own CA.
+      Trust inside a swarm derives from the secret store's pki root (see
+      docs/swarm/ca.md): every hive chains to it, so a per-hive CA is dead
+      weight. What this genuinely drops is trusting a hive whose root this
+      swarm does NOT own — another swarm's, or one keeping its own CA.
 
       Still set on: ${lib.concatStringsSep ", " withCaCert}
     '';