Revert "Add netcore 3.1 SDK action"

This reverts commit c957caa47b.

.github/workflows/build-and-test.yml

@@ -68,11 +68,6 @@ jobs:
               id: gitversion # step id used as reference for output values
               uses: gittools/actions/execute-gitversion@v0.3
 
-            - name: Install DotNet SDK
-            - uses: actions/setup-dotnet@v1
-              with:
-                  dotnet-version: "3.1.101" # SDK Version to use.
-
             - name: Build
               shell: pwsh
               run: ./ci-build.ps1 "${{steps.gitversion.outputs.nuGetVersion}}" "${{matrix.options.framework}}"