cockpit-beiboot: don't try to do explicit-superuser

This doesn't currently work in Cockpit Client — cockpit-ws always
returns the empty string as the password.  Don't even attempt it.

That prevents us from having to deal with the resulting transport
control messages and also prevents the delayed appearance of the
"Limited access" as described in cockpit-project#18927.

src/client/cockpit-beiboot

@@ -269,6 +269,7 @@ class SshBridge(Router):
         pass  # wait for the peer to do it first
 
     def do_init(self, message):
+        assert 'superuser' not in message  # see 'capabilities' below
         self.ssh_peer.write_control(**message)
 
 
@@ -280,6 +281,9 @@ async def run(args) -> None:
 
     try:
         message = await bridge.ssh_peer.start()
+        # https://github.com/cockpit-project/cockpit/issues/18927
+        if 'capabilities' in message:
+            message['capabilities']['explicit-superuser'] = False
         bridge.write_control(**dict(message, packages={p: None for p in bridge.packages.packages}))
         bridge.ssh_peer.thaw_endpoint()
     except ferny.InteractionError as exc: