Nodes: remove "Use Nodes" in Shader Editor for Object Materials

"Use Nodes" was removed in the compositor to simplify the compositing
workflow. This introduced a slight inconsistency with the Shader Node
Editor.

This PR removes "Use Nodes" for object materials.

For Line Style, no changes are planned (not sure how to preserve
compatibility yet).
This simplifies the state of objects; either they have a material or
they don't.

Backward compatibility:
- If Use Nodes is turned Off, new nodes are added to the node tree to
simulate the same material:
- DNA: Only `use_nodes` is marked deprecated
- Python API:
  - `material.use_nodes` is marked deprecated and will be removed in
6.0. Reading it always returns `True` and setting it has no effect.
  - `material.diffuse_color`, `material.specular` etc.. Are not used by
EEVEE anymore but are kept because they are used by Workbench.

Forward compatibility:
Always enable 'Use Nodes' when writing blend files.

Known Issues:
Some UI tests are failing on macOS

Pull Request: https://projects.blender.org/blender/blender/pulls/141278

tests/python/bl_node_group_interface.py

@@ -384,7 +384,6 @@ class ShaderNodeGroupInterfaceTest(AbstractNodeGroupInterfaceTest, NodeGroupInte
     def setUp(self):
         super().setUp()
         self.material = bpy.data.materials.new("test")
-        self.material.use_nodes = True
         self.main_tree = self.material.node_tree
 
     def test_invalid_socket_type(self):
@@ -476,7 +475,6 @@ class NodeTreeItemsIteratorTest(AbstractNodeGroupInterfaceTest, NodeGroupInterfa
     def setUp(self):
         super().setUp()
         self.material = bpy.data.materials.new("test")
-        self.material.use_nodes = True
         self.main_tree = self.material.node_tree
 
     # Regression test for changes while iterating over tree interface items (#143551).