Remove legacy query to find synced attachments

This commit removes the legacy queries to find synced
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

@@ -118,13 +118,7 @@ module Geo
     end
 
     def attachments_synced
-      if use_legacy_queries_for_selective_sync?
-        legacy_finder.attachments_synced
-      else
-        registries_for_attachments
-          .syncable
-          .merge(Geo::FileRegistry.synced)
-      end
+      registries_for_attachments.syncable.merge(Geo::FileRegistry.synced)
     end
 
     def attachments_migrated_local(except_file_ids:)

ee/app/finders/geo/legacy_attachment_registry_finder.rb

@@ -6,14 +6,6 @@ module Geo
       attachments.syncable
     end
 
-    def attachments_synced
-      legacy_inner_join_registry_ids(
-        syncable,
-        Geo::FileRegistry.attachments.synced.pluck_file_key,
-        Upload
-      )
-    end
-
     def attachments_migrated_local(except_file_ids:)
       registry_file_ids = Geo::FileRegistry.attachments.pluck_file_key - except_file_ids