T55454: removal of clay engine

The ClayEngine was introduced to test the blender2.8 architecture during
development. As currently we have the wanted features implemented with
matcaps we are going to remove the clay engine as it was never intended
to be an official releasable engine

Note: The test cases are never run. But when enabled will be skipped as
they were implemented over the Clay Engine

release/scripts/startup/bl_ui/properties_data_lattice.py

@@ -85,7 +85,7 @@ class DATA_PT_lattice(DataButtonsPanel, Panel):
 
 
 class DATA_PT_custom_props_lattice(DataButtonsPanel, PropertyPanel, Panel):
-    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_CLAY', 'BLENDER_EEVEE'}
+    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE'}
     _context_path = "object.data"
     _property_type = bpy.types.Lattice