Remove 贡献者数量增长图 in README

[yzeng25]

Error 报告

This repository is ONLY used to solve problems related to DOCS-CN.
For other issues (TiDB, TiKV, etc), please move to other repositories.

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

1. What is the document directory for the problem?

path: https://github.com/pingcap/docs-cn?tab=readme-ov-file#%E8%B4%A1%E7%8C%AE%E8%80%85%E6%95%B0%E9%87%8F%E5%A2%9E%E9%95%BF%E5%9B%BE

2. How would you like to improve it?

First discovered by @hugovk in api7/contributor-graph#231, then I validated and sure it is no longer working any more, raised an issue for record api7/contributor-graph#232.

In api7/contributor-graph#47, it states that pingcap/docs-cn 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.

[lilin90]

@yzeng25 Thanks for your kind reminder! I'll use the widget of active contributors provided by https://next.ossinsight.io/widgets to replace it.