text: Title: AUTHENTICATION_WHITELIST not working
  Body: ** Description **
  We are trying to override our DEFAULT_AUTHENTICATION_CLASSES that Swagger UI will use.  We have SessionAuthentication and TokenAuthentication set in our Django settings.   In our SPECTAULAR_SETTINGS we only want to use the TokenAuthentication, so we add it there as a single item list AUTHENTICATION_WHITELIST: ['rest_framework.authentication.TokenAuthentication'].  However swagger when loaded still shows both authentication methods.
  
  **To Reproduce**
  0.17.3
  
  ![image](https://user-images.githubusercontent.com/10231323/131222228-b8f45a9c-fcc4-40e2-844b-c47f6bed492a.png)
  
  ![image](https://user-images.githubusercontent.com/10231323/131222239-f7361c7d-869a-4699-8475-4ce60790d16e.png)
  
  
  
  **Expected behavior**
  As per the description in the settings docs, I expected only Token Authentication to appear?
  ![image](https://user-images.githubusercontent.com/10231323/131222249-37f9665b-fb36-4cf5-91d3-d6ab50d25b61.png)
  