[ci] Stop building classic test suites. (dotnet#7938)

Now that we no longer run any Classic test suites in main, we no longer need to build them with Xamarin.Android-Tests.sln.

The .NET test suites that we do use are built in Xamarin.Android.sln or built locally on the test agents with the installed XA test packages.

This saves about 7.5 minutes from the Mac Build.

build-tools/automation/yaml-templates/build-windows.yaml

@@ -45,11 +45,6 @@ stages:
       parameters:
         remove_dotnet: true
 
-    # xabuild still depends on .NET Core 3 or earlier
-    - template: use-dot-net.yaml
-      parameters:
-        version: 3.1.417
-
     # Downgrade the XA .vsix installed into the instance of VS that we are building with so that we don't restore/build against a test version.
     # The VS installer will attempt to resume any failed or partial installation before trying to downgrade Xamarin.Android.
     # VSIXInstaller.exe will exit non-zero when the downgrade attempt is a no-op, so we will allow this step to fail silently.
@@ -91,26 +86,6 @@ stages:
         displayName: Build Solution
         continueOnError: false
 
-    - task: MSBuild@1
-      displayName: msbuild create-vsix
-      inputs:
-        solution: build-tools\create-vsix\create-vsix.csproj
-        configuration: $(XA.Build.Configuration)
-        msbuildArguments: /p:CreateVsixContainer=True /p:ZipPackageCompressionLevel=Normal /bl:$(System.DefaultWorkingDirectory)\bin\Build$(XA.Build.Configuration)\dotnet-create-vsix.binlog
-
-    - task: MSBuild@1
-      displayName: msbuild xabuild
-      inputs:
-        solution: tools\xabuild\xabuild.csproj
-        configuration: $(XA.Build.Configuration)
-        msbuildArguments: /restore /bl:$(System.DefaultWorkingDirectory)\bin\Build$(XA.Build.Configuration)\dotnet-xabuild.binlog
-
-    - task: CmdLine@1
-      displayName: xabuild Xamarin.Android-Tests
-      inputs:
-        filename: bin\$(XA.Build.Configuration)\bin\xabuild.exe
-        arguments: Xamarin.Android-Tests.sln /restore /p:Configuration=$(XA.Build.Configuration) /bl:$(System.DefaultWorkingDirectory)\bin\Test$(XA.Build.Configuration)\dotnet-build-tests.binlog
-
     - template: install-apkdiff.yaml
 
     - template: run-nunit-tests.yaml

build-tools/automation/yaml-templates/commercial-build.yaml

@@ -62,15 +62,6 @@ steps:
   workingDirectory: ${{ parameters.xaSourcePath }}
   displayName: make jenkins
 
-# Build test assemblies
-- script: make all-tests CONFIGURATION=$(XA.Build.Configuration)
-  workingDirectory: ${{ parameters.xaSourcePath }}
-  displayName: make all-tests
-
-- template: install-microbuild-tooling.yaml
-  parameters:
-    condition: and(succeeded(), eq(variables['MicroBuildSignType'], 'Real'))
-
 # Restore needs to be executed first or MicroBuild targets won't be imported in time
 - task: MSBuild@1
   displayName: msbuild /t:Restore sign-content.proj