Anim: hide bone collection customprop editor on overrides

Blender's generic custom property panel doesn't support operating on
overridden data yet, so it's better to just hide the panel altogether.
Before this commit, the 'Custom Properties' panel was shown but empty.

scripts/startup/bl_ui/properties_data_armature.py

@@ -255,7 +255,17 @@ class DATA_PT_custom_props_bcoll(ArmatureButtonsPanel, PropertyPanel, Panel):
 
     @classmethod
     def poll(cls, context):
-        return context.armature and context.armature.collections.active
+        arm = context.armature
+        if not arm:
+            return False
+
+        is_lib_override = arm.id_data.override_library and arm.id_data.override_library.reference
+        if is_lib_override:
+            # This is due to a limitation in scripts/modules/rna_prop_ui.py; if that
+            # limitation is lifted, this poll function should be adjusted.
+            return False
+
+        return arm.collections.active
 
 
 classes = (