CI: Don't specify a Python version on Windows

We support both Python 2.6+ and Python 3.5+.
The exact version doesn't matter.

Author: DeeDeeG

script/vsts/platforms/templates/preparation.yml

@@ -37,11 +37,6 @@ steps:
     displayName: Setting globally used env variables
 
   # Windows Specific
-  - task: UsePythonVersion@0
-    inputs:
-      versionSpec: '3.8'
-    condition: eq(variables['Agent.OS'], 'Windows_NT')
-
   - script: |
       cd script\vsts
       npm install