chore: remove dead link to v3 docs

Author: RooRay

README.md

@@ -5,9 +5,6 @@
 This is the source for the Discord Tickets v4 documentation (and marketing) website.
 You can view the latest build of the site [here](https://discordtickets.app/).
 
-> **Note**
-> For the v3 documentation, visit [v3.discordtickets.app](https://v3.discordtickets.app) or look at the `v3.1` branch.
-
 ## Contributing
 
 See [CONTRIBUTING.md](./CONTRIBUTING.md)