Remove legacy query to find registries for job artifacts

This commit removes the legacy queries to find registries
for job artifacts since we made Foreign Data Wrapper (FDW)
a hard requirement for Geo on GitLab 12.0.

ee/app/finders/geo/job_artifact_registry_finder.rb

@@ -108,15 +108,11 @@ module Geo
     end
 
     def registries_for_job_artifacts
-      if use_legacy_queries_for_selective_sync?
-        legacy_finder.registries_for_job_artifacts
-      else
       fdw_geo_node
         .job_artifacts
         .inner_join_job_artifact_registry
         .syncable
     end
-    end
 
     def job_artifacts_unsynced(except_artifact_ids:)
       fdw_geo_node

ee/app/finders/geo/legacy_job_artifact_registry_finder.rb

@@ -25,16 +25,5 @@ module Geo
         Ci::JobArtifact
       )
     end
-
-    def registries_for_job_artifacts
-      return Geo::JobArtifactRegistry.all unless selective_sync?
-
-      legacy_inner_join_registry_ids(
-        Geo::JobArtifactRegistry.all,
-        current_node.job_artifacts.pluck_primary_key,
-        Geo::JobArtifactRegistry,
-        foreign_key: :artifact_id
-      )
-    end
   end
 end