Remove duplicate definitions in readme.md

Author: wooorm

readme.md

@@ -209,12 +209,8 @@ Thanks to [**@nwtn**](https://github.com/nwtn),
 
 [latest]: https://github.com/syntax-tree/nlcst/releases/tag/1.0.1
 
-[retext]: https://github.com/wooorm/retext
-
 [vfile]: https://github.com/vfile/vfile
 
-[unist]: https://github.com/syntax-tree/unist
-
 [hast]: https://github.com/syntax-tree/hast
 
 [mdast]: https://github.com/syntax-tree/mdast