delete sub.title.rename_label (#7881)

Ref #7871 
Ref #5954

Changes proposed in this pull request:

- `sub.title.rename_label ` has been not used anymore since #5954 (Version V1.24.0)
- `sub.title.rename_label ` deleted

app/i18n/ja/sub.php

@@ -301,7 +301,6 @@ return array(
 		'add_opml_category' => 'OPMLカテゴリ名',
 		'delete_label' => 'ラベルの削除',
 		'feed_management' => 'RSSフィードの管理',
-		'rename_label' => 'ラベルの名前変更',
 		'subscription_tools' => '購読ツール',
 	),
 );