Cleanup: Fully remove legacy displacement_method property

The commit[1] to unify the displacement method for both EEVEE and Cycles
left this, now unused, property in place.

[1] a001cf9f2b

Pull Request: https://projects.blender.org/blender/blender/pulls/115738

intern/cycles/blender/addon/properties.py

@@ -43,12 +43,6 @@ enum_feature_set = (
      1),
 )
 
-enum_displacement_methods = (
-    ('BUMP', "Bump Only", "Bump mapping to simulate the appearance of displacement"),
-    ('DISPLACEMENT', "Displacement Only", "Use true displacement of surface only, requires fine subdivision"),
-    ('BOTH', "Displacement and Bump", "Combination of true displacement and bump mapping for finer detail"),
-)
-
 enum_bvh_layouts = (
     ('BVH2', "BVH2", "", 1),
     ('EMBREE', "Embree", "", 4),
@@ -1047,13 +1041,6 @@ class CyclesMaterialSettings(bpy.types.PropertyGroup):
         min=0.001, max=1000.0, soft_min=0.1, soft_max=10.0, precision=4
     )
 
-    displacement_method: EnumProperty(
-        name="Displacement Method",
-        description="Method to use for the displacement",
-        items=enum_displacement_methods,
-        default='BUMP',
-    )
-
     @classmethod
     def register(cls):
         bpy.types.Material.cycles = PointerProperty(