Can't remove built-in tax providers

[j-mok]

In VC v2 we could replace all existing tax providers by implementing VirtoCommerce.Domain.Tax.Services.ITaxService.GetAllTaxProviders(). With version 3 this is no longer possible. The new ITaxProviderRegistrar provides only a method to register a new tax provider, there is no way to override or unregister exiting ones. The underlying AbstractTypeFactory<TaxProvider> in RegisterTaxProvider's stock implementation does not make thing much easier - it can't have types unregistered either, just overridden, which is some sort of a workaround for now. Still we don't want to rely on implementation details and make brittle code, we'd very much rather see a proper UnregisterAllTaxProviders() or something along the lines in ITaxProviderRegistrar.