--- delete(key)
--- delete(key) {|key| ... }

    key ФϢޤ줿֤ͤޤ
    key бͤ¸ߤʤˤ nil ֤ޤ

    ֥åͿ줿ˤ key ˥ޥåΤ
    äɾη̤֤ޤ

