[release/6.0] Don't clear NuGet caches at all (#37122)

- backport of #36898

Don't clear NuGet caches at all (#36898)

  - `$use_global_nuget_cache` and `$useGlobalNuGetCache` have default values in our pipelines
  - therefore, cache is always within `$(System.DefaultWorkingDirectory)` and should not exist at job start