ci: fix multiple checkouts on azure

We were checking out the rustc repo multiple times on auto macOS, and
that was causing an error on the Azure side since multiple checkouts are
apparently not supported. This removes the extra checkout.

Author: pietroalbini

Diff for: .azure-pipelines/auto.yml

@@ -153,8 +153,6 @@ jobs:
   pool:
     vmImage: macos-10.13
   steps:
-  - checkout: self
-    fetchDepth: 2
   - template: steps/run.yml
   strategy:
     matrix: