Remove backup_labels temporary table

What does this MR do?

Remove backup_labels table, previously used to remove duplicated labels from database on background migrations.

Migration output

Up

== 20210222185538 RemoveBackupLabelsForeignKeys: migrating ====================
-- foreign_keys(:backup_labels)
   -> 0.0027s
-- remove_foreign_key(:backup_labels, :projects)
   -> 0.0037s
-- foreign_keys(:backup_labels)
   -> 0.0019s
-- remove_foreign_key(:backup_labels, :namespaces)
   -> 0.0041s
== 20210222185538 RemoveBackupLabelsForeignKeys: migrated (0.0174s) ===========

== 20210222192144 RemoveBackupLabelsTable: migrating ==========================
-- drop_table(:backup_labels)
   -> 0.0056s
== 20210222192144 RemoveBackupLabelsTable: migrated (0.0057s) =================

Down

  == 20210222192144 RemoveBackupLabelsTable: reverting ==========================
-- create_table(:backup_labels, {:id=>false})
   -> 0.0073s
-- execute("ALTER TABLE backup_labels ADD PRIMARY KEY (id)")
   -> 0.0036s
-- add_index(:backup_labels, [:group_id, :project_id, :title], {:name=>"backup_labels_group_id_project_id_title_idx", :unique=>true})
   -> 0.0060s
-- add_index(:backup_labels, [:group_id, :title], {:where=>"project_id = NULL::integer", :name=>"backup_labels_group_id_title_idx"})
   -> 0.0044s
-- add_index(:backup_labels, :project_id, {:name=>"backup_labels_project_id_idx"})
   -> 0.0079s
-- add_index(:backup_labels, :template, {:name=>"backup_labels_template_idx", :where=>"template"})
   -> 0.0054s
-- add_index(:backup_labels, :title, {:name=>"backup_labels_title_idx"})
   -> 0.0042s
-- add_index(:backup_labels, [:type, :project_id], {:name=>"backup_labels_type_project_id_idx"})
   -> 0.0045s
== 20210222192144 RemoveBackupLabelsTable: reverted (0.0438s) =================

== 20210222185538 RemoveBackupLabelsForeignKeys: reverting ====================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:backup_labels)
   -> 0.0026s
-- execute("ALTER TABLE backup_labels\nADD CONSTRAINT fk_5f48959a88\nFOREIGN KEY (project_id)\nREFERENCES projects (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0015s
-- execute("SET statement_timeout TO 0")
   -> 0.0009s
-- execute("ALTER TABLE backup_labels VALIDATE CONSTRAINT fk_5f48959a88;")
   -> 0.0041s
-- execute("RESET ALL")
   -> 0.0006s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:backup_labels)
   -> 0.0022s
-- execute("ALTER TABLE backup_labels\nADD CONSTRAINT fk_c530ff0848\nFOREIGN KEY (group_id)\nREFERENCES namespaces (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0010s
-- execute("ALTER TABLE backup_labels VALIDATE CONSTRAINT fk_c530ff0848;")
   -> 0.0025s
== 20210222185538 RemoveBackupLabelsForeignKeys: reverted (0.0274s) ===========

related to #227753 (closed)