Simplify stage_id migration as we now use relations

lib/gitlab/background_migration/migrate_build_stage_id_reference.rb

@@ -2,19 +2,14 @@ module Gitlab
   module BackgroundMigration
     class MigrateBuildStageIdReference
       def perform(start_id, stop_id)
-        scope = if stop_id.to_i.nonzero?
-                  "ci_builds.id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}"
-                else
-                  "ci_builds.id >= #{start_id.to_i}"
-                end
-
         sql = <<-SQL.strip_heredoc
-          UPDATE "ci_builds"
-            SET "stage_id" =
+          UPDATE ci_builds
+            SET stage_id =
               (SELECT id FROM ci_stages
                 WHERE ci_stages.pipeline_id = ci_builds.commit_id
                 AND ci_stages.name = ci_builds.stage)
-          WHERE #{scope} AND "ci_builds"."stage_id" IS NULL
+          WHERE ci_builds.id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}
+            AND ci_builds.stage_id IS NULL
         SQL
 
         ActiveRecord::Base.connection.execute(sql)