Cancel overriden url of Upgrade button

Problem to solve

In past 2 months we had overriden the url of Upgrade button to https://about.gitlab.cn/upgrade-plan.

But for now, we need to cancel overriden url of Upgrade button since https://about.gitlab.cn/upgrade-plan will be offline sooner or later.

proposal

1. remove the overriden mothod of Upgrade button
2. override use_new_purchase_flow? to return false see https://gitlab.com/gitlab-jh/gitlab/-/blob/main-jh/jh/app/helpers/jh/billing_plans_helper.rb#L17-20

CC @qianzhangxa