ci: remove Windows Server 2016 from builds on Azure Pipelines (#652)

The images with Windows Server 2016 / Visual Studio 2017 have
been deprecated by Microsoft and are in the process of being
removed completely, so that no further builds with that image
will be possible.

For reference, see
* <https://devblogs.microsoft.com/devops/hosted-pipelines-image-deprecation/>
* <actions/runner-images#5238>

Since it seems that Microsoft is basically using the same build
environments on both Azure and GitHub Actions when it comes to
Windows and Visual Studio, I decided to remove those jobs instead
of updating them to a newer version of the image. The tests that
would be possible with newer images on Azure are basically what
we are already doing with GitHub Actions.

Author: striezel

.azure-pipelines.yml

@@ -53,41 +53,6 @@ jobs:
         parameters:
           cxxver: '14'
 
-  - job: 'win2016_vs2017_cxx14_cmake'
-    pool:
-      vmImage: 'vs2017-win2016'
-    steps:
-      - task: UsePythonVersion@0
-        displayName: 'Setup Python'
-        inputs:
-          versionSpec: '3.6'
-          addToPath: true
-          architecture: 'x64'
-      - template: .ci/azure-pipelines/steps-check-cmake.yml
-      - template: .ci/azure-pipelines/steps-install-conan.yml
-      - template: .ci/azure-pipelines/steps-install-boost.yml
-      - template: .ci/azure-pipelines/steps-cmake-build-and-test.yml
-        parameters:
-          use_conan: 'ON'
-
-  - job: 'win2016_vs2017_cxx17_cmake'
-    pool:
-      vmImage: 'vs2017-win2016'
-    steps:
-      - task: UsePythonVersion@0
-        displayName: 'Setup Python'
-        inputs:
-          versionSpec: '3.6'
-          addToPath: true
-          architecture: 'x64'
-      - template: .ci/azure-pipelines/steps-check-cmake.yml
-      - template: .ci/azure-pipelines/steps-install-conan.yml
-      - template: .ci/azure-pipelines/steps-install-boost.yml
-      - template: .ci/azure-pipelines/steps-cmake-build-and-test.yml
-        parameters:
-          cxxver: '17'
-          use_conan: 'ON'
-
   - job: 'macos1015_xcode11_cmake'
     pool:
       vmImage: 'macOS-10.15'