UI: Remove left indent from shapekey tree view

Set `is_flat_ = true` for shape key tree view, that way add_intent()
will skip the condition that includes extra width on left (see `is_flat_list`).

Pull Request: https://projects.blender.org/blender/blender/pulls/140180

source/blender/editors/object/interface_template_shape_key_tree.cc

@@ -33,7 +33,10 @@ class ShapeKeyTreeView : public ui::AbstractTreeView {
   Object &object_;
 
  public:
-  ShapeKeyTreeView(Object &ob) : object_(ob){};
+  ShapeKeyTreeView(Object &ob) : object_(ob)
+  {
+    is_flat_ = true;
+  };
 
   void build_tree() override;
 };