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

    key бĶѿޤ줿Ķѿ
    ͤ ޤkey бĶѿ¸ߤʤˤ 
    nil ֤ޤ

    ֥åͿ줿ˤ key ˥ޥåΤʤä
    ɾޤ

