update python autorest pr (Azure#30328)

Author: iscai-msft

eng/pipelines/autorest_checks.yml

@@ -17,7 +17,6 @@ variables:
   NodeVersion: '18.x'
   PythonVersion: '3.7'
   auto_rest_clone_url: 'https://github.com/Azure/autorest.python.git'
-  repo_branch: 'autorestv3'
   source_path_azure_core: 'sdk/core/azure-core'
   source_path_azure_mgmt_core: 'sdk/core/azure-mgmt-core'
 
@@ -40,10 +39,9 @@ jobs:
           versionSpec: $(PythonVersion)
 
       - script: |
-          npm install -g pnpm@7.27.1
+          npm install -g pnpm
           git clone $(auto_rest_clone_url)
           cd autorest.python
-          git checkout $(repo_branch)
           pnpm install
         displayName: 'Prepare Environment'