Move ZenTao integration to JH

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What does this MR do and why?

Move ZenTao integration to JH

Related issue: #377825

JH MR here: https://jihulab.com/gitlab-cn/gitlab/-/merge_requests/2448

We've done these jobs in current MR:

• Remove related code
• Remove related metric
• Remove documentation: https://docs.gitlab.com/ee/user/project/integrations/zentao.html

There're two questions about the issue requests:

• Issue mentioned Delete database records, need it be done in this MR or not?
• Issue mentioned Add removal entry, and I found it seems already be done in the online doc, so the MR ignore it for now. Is it OK?