Remove broken/useless Readme link from repo sidebar

Description

In https://github.com/refined-github/refined-github/issues/9908 I noticed that our feature was interfering with the link and we fixed it. It seems that the link is now just broken even without Refined GitHub. I think we should just drop this useless link anyway

It should be done in clean-repo-sidebar

https://github.com/user-attachments/assets/dd4b151c-575b-41d8-8d33-ac9bd8cf65bc

Example URLs

https://github.com/refined-github/refined-github