hivectl, hive-c0re: remove dead matrix create-user/promote-user/reset-password

Human matrix accounts come from SSO, not hivectl. Matrix homeserver
admin will come from authelia's admins group (sync tracked in #4585);
password reset moves to swarm level (#4798). promote-user and
reset-password were already broken from the hive: the hive's sender
account has no admin sender to call the admin room with, only the
swarm's does.

Removes the three hivectl matrix verbs, their HostRequest variants,
their hive-c0re handlers, and the admin-room helpers (discover room id,
send-and-poll, event-id extraction, password/success parsing) that
only they used. sync-admin and invite are unchanged.

Refs #4585

nix/host-modules/hive-matrix.nix

@@ -200,9 +200,8 @@ let
   # knows about itself.
   ctlHomeserverUrl = if cfg.gatewayHost == null then "" else "https://${toString cfg.gatewayHost}";
 
-  # Every local user this hive may provision — agents, `@hive-<hive>:` itself, and
-  # the operator accounts `hivectl matrix create-user` makes, which is the
-  # whole matrix localpart charset.
+  # Every local user this hive may provision — agents and `@hive-<hive>:`
+  # itself — which is the whole matrix localpart charset.
   #
   # ⚠️ Anchored deliberately: tuwunel compiles a namespace into a `RegexSet`
   # and asks it for a MATCH, not a full match, so an unanchored