fix: remove opencollective links in README

Author: hsluoyz

README.md

@@ -245,27 +245,6 @@ Please read the [contributing guide](CONTRIBUTING.md).
 This project exists thanks to all the people who contribute.
 <a href="https://github.com/casbin/casbin/graphs/contributors"><img src="https://opencollective.com/casbin/contributors.svg?width=890&button=false" /></a>
 
-## Backers
-
-Thank you to all our backers! 🙏 [[Become a backer](https://opencollective.com/casbin#backer)]
-
-<a href="https://opencollective.com/casbin#backers" target="_blank"><img src="https://opencollective.com/casbin/backers.svg?width=890"></a>
-
-## Sponsors
-
-Support this project by becoming a sponsor. Your logo will show up here with a link to your website. [[Become a sponsor](https://opencollective.com/casbin#sponsor)]
-
-<a href="https://opencollective.com/casbin/sponsor/0/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/0/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/1/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/1/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/2/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/2/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/3/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/3/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/4/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/4/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/5/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/5/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/6/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/6/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/7/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/7/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/8/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/8/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/9/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/9/avatar.svg"></a>
-
 ## Star History
 
 [![Star History Chart](https://api.star-history.com/svg?repos=casbin/casbin&type=Date)](https://star-history.com/#casbin/casbin&Date)