fix(CONTRIBUTING): remove section about beta releases (#2109)

We don't have beta releases yet (see the [script](https://github.com/algolia/instantsearch.js/blob/develop/scripts/release.sh#L85-L92) and #2056), so the contributing section shouldn't be added yet.

We will put this back when -b works, and we've found a strategy for making it work with jsDeliver

Author: Haroenv

CONTRIBUTING.md

@@ -57,14 +57,6 @@ Files are automatically formatted with prettier.
 npm run release
 ```
 
-### Beta release
-
-```sh
-npm run release -- -b # or --beta
-```
-
-Append `-beta.x` where x is a number to the version for beta, so 4.0.0-beta.2 for example.
-
 ## Update docs
 
 Documentation website is automatically updated by Travis when master is merged.