Remove XFrameOptionsMiddleware

The XFrameOptionsMiddleware middleware sets the X-Frame-Options header, which we already do in our nginx instance.

Previous behaviour

$ curl -I https://thalia.nu/
...
x-frame-options: SAMEORIGIN
...
x-frame-options: SAMEORIGIN
...

New behaviour

$ curl -I https://thalia.nu/
...
x-frame-options: SAMEORIGIN
...