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

docs/swarm/services.md

@@ -20,8 +20,7 @@ sections below don't repeat it, so a service that stops deriving is a
 visible difference rather than one more paragraph saying the same thing.
 
 `singleHostSwarm` is the all-on-one-box mode above it. It defaults
-`deploy.allSwarmServices`, the swarm CA (`swarm.ca.autoConfigure`, generated
-on this host), the swarm controller (`deploy.swarm-controller.enable`), the
+`deploy.allSwarmServices`, the swarm controller (`deploy.swarm-controller.enable`), the
 host's `/etc/hosts` entries for the names it serves
 (`gateway.localHostsEntry`), the queue's auth-callout keys
 (`deploy.nats.autoGenerateCallout`) and where the secret store's bootstrap