Remove 贡献者数量增长图 in README

[yzeng25]

Error Report

This repository is ONLY used to solve issues related to TiDB Data Migration (DM) documentation.
For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. What is the URL/path of the document related to this issue?

https://github.com/pingcap/docs-dm?tab=readme-ov-file#contributors-over-time

2. How would you like to improve it?

Though this repo seems to be no longer maintained any more, I still need to point it out.

In api7/contributor-graph#47, it states that pingcap/docs-dm repo is using it and I think you should remove this section in your README given that no one can ensure when it is gonna work again. Also see pingcap/docs-cn#19086 for detail,