Remove unused middlewares (#11722)
- We no longer hit APIs from our main domain from docs pages,
so there is no need to not se cookies.
- SECURE_REFERRER_POLICY is a built-in Django setting
now https://docs.djangoproject.com/en/4.2/ref/middleware/#referrer-policy.
Closes https://github.com/readthedocs/readthedocs.org/issues/11692
