Remove legacy query to find attachments that sync failed

This commit removes the legacy queries to find attachments
that sync failed since we made Foreign Data Wrapper (FDW)
a hard requirement for Geo on GitLab 12.0.

ee/app/finders/geo/attachment_registry_finder.rb

@@ -137,13 +137,7 @@ module Geo
     end
 
     def attachments_failed
-      if use_legacy_queries_for_selective_sync?
-        legacy_finder.attachments_failed
-      else
-        registries_for_attachments
-          .syncable
-          .merge(Geo::FileRegistry.failed)
-      end
+      registries_for_attachments.syncable.merge(Geo::FileRegistry.failed)
     end
 
     def attachments_synced_missing_on_primary

ee/app/finders/geo/legacy_attachment_registry_finder.rb

@@ -26,14 +26,6 @@ module Geo
       )
     end
 
-    def attachments_failed
-      legacy_inner_join_registry_ids(
-        syncable,
-        Geo::FileRegistry.attachments.failed.pluck_file_key,
-        Upload
-      )
-    end
-
     def attachments_synced_missing_on_primary
       legacy_inner_join_registry_ids(
         syncable,