code: public Observable<Void> purgeDeletedKeyAsync(String vaultBaseUrl, String keyName) {
          return purgeDeletedKeyWithServiceResponseAsync(vaultBaseUrl, keyName).map(new Func1<ServiceResponse<Void>, Void>() {
              @Override
              public Void call(ServiceResponse<Void> response) {
                  return response.body();
              }
          });
      }
docs: Permanently deletes the specified key.
  The Purge Deleted Key operation is applicable for soft-delete enabled vaults. While the operation can be invoked on any vault, it will return an error if invoked on a non soft-delete enabled vault. This operation requires the keys/purge permission.
  
  @param vaultBaseUrl The vault name, for example https://myvault.vault.azure.net.
  @param keyName The name of the key
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceResponse} object if successful.