Remove .net 6 runtime requirement from dotnet-helix-service

[chcosta]

Dotnet-helix-service has a requirement on the .NET 6 runtime because it utilizes secret-manager, which is not yet updated to .NET 8 support. AFTER https://github.com/dotnet/dnceng/issues/1430 is completed and secret-manager targets .NET 8, we should remove these additional runtime installs from the dotnet-helix-service build.

Sample .NET 6 runtime install - https://dev.azure.com/dnceng/internal/_git/dotnet-helix-service?path=/eng/deploy.yaml&version=GBmain&line=30&lineEnd=34&lineStartColumn=1&lineEndColumn=23&lineStyle=plain&_a=contents

There are more locations in the repo which also install 6.0 and should be updated.