Remove legacy finder for attachments

This commit removes the legacy finder for attachments since
we made Foreign Data Wrapper (FDW) a hard requirement for
Geo on GitLab 12.0.

ee/app/finders/geo/attachment_registry_finder.rb

@@ -23,9 +23,7 @@ module Geo
     end
 
     def syncable
-      if use_legacy_queries_for_selective_sync?
-        legacy_finder.syncable
-      elsif selective_sync?
+      if selective_sync?
         attachments.syncable
       else
         Upload.syncable
@@ -79,12 +77,6 @@ module Geo
 
     private
 
-    # rubocop:disable CodeReuse/Finder
-    def legacy_finder
-      @legacy_finder ||= Geo::LegacyAttachmentRegistryFinder.new(current_node: current_node)
-    end
-    # rubocop:enable CodeReuse/Finder
-
     def fdw_geo_node
       @fdw_geo_node ||= Geo::Fdw::GeoNode.find(current_node.id)
     end

ee/app/finders/geo/legacy_attachment_registry_finder.rb

-# frozen_string_literal: true
-
-module Geo
-  class LegacyAttachmentRegistryFinder < RegistryFinder
-    def syncable
-      attachments.syncable
-    end
-
-    private
-
-    def attachments
-      current_node.attachments
-    end
-  end
-end

ee/spec/finders/geo/attachment_registry_finder_spec.rb

@@ -503,56 +503,6 @@ describe Geo::AttachmentRegistryFinder, :geo do
       stub_fdw_disabled
     end
 
-    describe '#count_syncable' do
-      let!(:upload_1) { create(:upload, model: synced_group) }
-      let!(:upload_2) { create(:upload, model: unsynced_group) }
-      let!(:upload_3) { create(:upload, :issuable_upload, model: synced_project_in_nested_group) }
-      let!(:upload_4) { create(:upload, model: unsynced_project) }
-      let!(:upload_5) { create(:upload, :personal_snippet_upload) }
-
-      it 'counts attachments' do
-        expect(subject.count_syncable).to eq 5
-      end
-
-      it 'ignores remote attachments' do
-        upload_1.update!(store: ObjectStorage::Store::REMOTE)
-
-        expect(subject.count_syncable).to eq 4
-      end
-
-      context 'with selective sync by namespace' do
-        before do
-          secondary.update!(selective_sync_type: 'namespaces', namespaces: [synced_group])
-        end
-
-        it 'counts attachments' do
-          expect(subject.count_syncable).to eq 3
-        end
-
-        it 'ignores remote attachments' do
-          upload_1.update!(store: ObjectStorage::Store::REMOTE)
-
-          expect(subject.count_syncable).to eq 2
-        end
-      end
-
-      context 'with selective sync by shard' do
-        before do
-          secondary.update!(selective_sync_type: 'shards', selective_sync_shards: ['broken'])
-        end
-
-        it 'counts attachments' do
-          expect(subject.count_syncable).to eq 3
-        end
-
-        it 'ignores remote attachments' do
-          upload_4.update!(store: ObjectStorage::Store::REMOTE)
-
-          expect(subject.count_syncable).to eq 2
-        end
-      end
-    end
-
     describe '#count_registry' do
       let!(:upload_1) { create(:upload, model: synced_group) }
       let!(:upload_2) { create(:upload, model: unsynced_group) }