[build] we don't need to install a system .NET 6 (#594)

CI is currently downloading/installing .NET 6 twice, once via
`UseDotNet` in `.yml` and again via `DotNet.csproj`.

I don't think we actually need to do call `UseDotNet`, I was able to
run `eng\package.ps1` without .NET 6 installed.

eng/pipelines/handlers.yml

@@ -194,11 +194,6 @@ stages:
                   /usr/libexec/PlistBuddy -c "add :AppleSdkRoot string $(dirname $(dirname $(xcode-select -p)))" ~/Library/Preferences/Xamarin/Settings.plist || true
                   cat ~/Library/Preferences/Xamarin/Settings.plist || true
                 displayName: configure vsmac xcode
-            - ${{ if eq(BuildPlatform.name, 'windows') }}:
-              - task: UseDotNet@2
-                inputs:
-                  version: 6.x
-                  includePreviewVersions: true
             - pwsh: ./eng/package.ps1 -configuration Release
               displayName: pack nugets
               errorActionPreference: stop