Cleanup: Remove Dots Stroke material from USD's empty.blend test file

This extra material in what should be an empty file often makes some
checks and text comparisons needlessly more confusing.

Pull Request: https://projects.blender.org/blender/blender/pulls/139228

tests/files/usd/compare/reference/usd_anim_camera_props.txt

@@ -291,19 +291,6 @@
   - fov 0.691 (h 0.691 v 0.400)
   - sensor 36.0x20.2 shift 0.000,0.000
 
-==== Materials: 1
-- Mat 'Dots Stroke'
-  - base color (0.800, 0.800, 0.800)
-  - specular ior 0.500
-  - specular tint (0.000, 0.000, 0.000)
-  - roughness 0.400
-  - metallic 0.000
-  - ior 1.000
-  - viewport diffuse (0.800, 0.800, 0.800, 1.000)
-  - viewport specular (1.000, 1.000, 1.000), intensity 0.500
-  - viewport metallic 0.000, roughness 0.400
-  - backface False probe True shadow False
-
 ==== Actions: 4
 - Action 'CamAAction' curverange:(1.0 .. 5.0) layers:1
 - ActionLayer Layer strips:1

tests/python/bl_usd_import_test.py

@@ -1691,13 +1691,13 @@ class USDImportTest(AbstractUSDTest):
                 self.assertEqual(image.tiles[tile].size[1], size)
 
         def check_materials():
-            self.assertEqual(len(bpy.data.materials), 7)  # +1 because of the "Dots Stroke" material
             self.assertTrue("Clip_With_LessThanInvert" in bpy.data.materials)
             self.assertTrue("Clip_With_Round" in bpy.data.materials)
             self.assertTrue("Material" in bpy.data.materials)
             self.assertTrue("NormalMap" in bpy.data.materials)
             self.assertTrue("NormalMap_Scale_Bias" in bpy.data.materials)
             self.assertTrue("Transforms" in bpy.data.materials)
+            self.assertEqual(len(bpy.data.materials), 6)
 
         # Reload the empty file and import back in using IMPORT_PACK
         bpy.ops.wm.open_mainfile(filepath=str(self.testdir / "empty.blend"))