[docs] Keep showing next under the subdomain

I don't think that we need this. It was changed in #43447 but it's like the npm tag.
Having a banner on this URL should be enough to signal to people that they are not
on the latest version.

docs/public/_redirects

@@ -7,9 +7,6 @@
 
 /size-snapshot https://s3.eu-central-1.amazonaws.com/mui-org-ci/artifacts/master/latest/size-snapshot.json 200
 
-# To add when we finish work on v6
-# https://next.mui.com/* https://mui.com/:splat 301!
-
 # For links that we can't edit later on, for example hosted in the code published on npm
 /r/styles-instance-warning /material-ui/getting-started/faq/#i-have-several-instances-of-styles-on-the-page 302
 /r/caveat-with-refs-guide /material-ui/guides/composition/#caveat-with-refs 302