Geo - Remove finder_class method from registry models

The following discussion from !32695 (merged) should be addressed:

• @mkozono started a discussion:

  thought: Tangential to this MR, the finder has been bothering me.

  1. The current_node_id arg is irrelevant, it's always "this" node
  2. The only registries that can be queried are the ones on "this" node.
  3. It might be simpler if we didn't need to know about finder_class and just called registry_class.find_registry_differences.

  I wonder if the finder + current_node dynamic is now just a historical thing that we can leave behind, if not now, then perhaps after removing FDW... 🤔