remove the concept of cluster user (at least the current implementation)

internal/cli/config/cluster.go

@@ -1,10 +1,6 @@
 package config
 
-import "uncloud/internal/secret"
-
 type Cluster struct {
 	Name        string              `toml:"-"`
 	Connections []MachineConnection `toml:"connections"`
-	// UserPrivateKey is the user's WireGuard private key used to connect to cluster machines.
-	UserPrivateKey secret.Secret `toml:"user_private_key"`
 }