Remove Windows Server 2016 from CI (#1626)

The image has been deprecated and will soon be removed from Azure
DevOps. Furthermore Microsoft's support for Windows Server 2016 itself
ends in January 2022 (less than two months away).

Author: andyleejordan

.vsts-ci/azure-pipelines-ci.yml

@@ -9,15 +9,6 @@ variables:
     value: 'true'
 
 jobs:
-- job: PS51_Win2016
-  displayName: PowerShell 5.1 - Windows Server 2016
-  pool:
-    vmImage: vs2017-win2016
-  steps:
-  - template: templates/ci-general.yml
-    parameters:
-      pwsh: false
-
 - job: PS51_Win2019
   displayName: PowerShell 5.1 - Windows Server 2019
   pool: