Update doc.

Author: nicolasstucki

docs/docs/contributing/workflow.md

@@ -11,8 +11,6 @@ This document details common workflow patterns when working with Dotty.
 # Start by cloning the repository:
 git clone --recursive https://github.com/lampepfl/dotty.git
 cd dotty
-# Clone dotty-compatible stdlib. Needed for running the test suite.
-git clone -b dotty-library https://github.com/DarkDimius/scala.git scala-scala
 ```
 
 ## Compiling files with dotc ##