Clean up unused EE background migration overrides

These override modules for EE background migrations are no longer
used, so we can clean them up.

Closes https://gitlab.com/gitlab-org/gitlab/issues/32927

ee/lib/ee/gitlab/background_migration/populate_import_state.rb

-# frozen_string_literal: true
-
-module EE
-  module Gitlab
-    module BackgroundMigration
-      module PopulateImportState
-        extend ::Gitlab::Utils::Override
-
-        override :move_attributes_data_to_import_state
-        def move_attributes_data_to_import_state(start_id, end_id)
-          Rails.logger.info("#{self.class.name} - Moving import attributes data to project mirror data table: #{start_id} - #{end_id}") # rubocop:disable Gitlab/RailsLogger
-
-          ActiveRecord::Base.connection.execute <<~SQL
-            INSERT INTO project_mirror_data (project_id, status, jid, last_update_at, last_successful_update_at, last_error)
-            SELECT id, import_status, import_jid, mirror_last_update_at, mirror_last_successful_update_at, import_error
-            FROM projects
-            WHERE projects.import_status != 'none'
-            AND projects.id BETWEEN #{start_id} AND #{end_id}
-            AND NOT EXISTS (
-              SELECT id
-              FROM project_mirror_data
-              WHERE project_id = projects.id
-            )
-          SQL
-
-          ActiveRecord::Base.connection.execute <<~SQL
-            UPDATE projects
-            SET import_status = 'none'
-            WHERE import_status != 'none'
-            AND id BETWEEN #{start_id} AND #{end_id}
-          SQL
-        end
-      end
-    end
-  end
-end

ee/lib/ee/gitlab/background_migration/rollback_import_state_data.rb

-# frozen_string_literal: true
-
-module EE
-  module Gitlab
-    module BackgroundMigration
-      module RollbackImportStateData
-        extend ::Gitlab::Utils::Override
-
-        override :move_attributes_data_to_project
-        def move_attributes_data_to_project(start_id, end_id)
-          Rails.logger.info("#{self.class.name} - Moving import attributes data to projects table: #{start_id} - #{end_id}") # rubocop:disable Gitlab/RailsLogger
-
-          ActiveRecord::Base.connection.execute <<~SQL
-            UPDATE projects
-            SET
-              import_status = project_mirror_data.status,
-              import_jid = project_mirror_data.jid,
-              mirror_last_update_at = project_mirror_data.last_update_at,
-              mirror_last_successful_update_at = project_mirror_data.last_successful_update_at,
-              import_error = project_mirror_data.last_error
-            FROM project_mirror_data
-            WHERE project_mirror_data.project_id = projects.id
-            AND project_mirror_data.id BETWEEN #{start_id} AND #{end_id}
-          SQL
-        end
-      end
-    end
-  end
-end