[#1431] engine: Delete unused constants

Signed-off-by: Alexander Chuprov <a.chuprov@yadro.com>

cmd/frostfs-cli/modules/ape_manager/add_chain.go

@@ -31,11 +31,10 @@ const (
 )
 
 const (
-	defaultNamespace = ""
-	namespaceTarget  = "namespace"
-	containerTarget  = "container"
-	userTarget       = "user"
-	groupTarget      = "group"
+	namespaceTarget = "namespace"
+	containerTarget = "container"
+	userTarget      = "user"
+	groupTarget     = "group"
 )
 
 var errUnknownTargetType = errors.New("unknown target type")