Remove x-forwarded-prefix from trusted proxy headers

Fix for:

  ValueError: Received unknown trusted_proxy_headers value
  (x-forwarded-prefix) expected one of x-forwarded-for,
  x-forwarded-host, forwarded, x-forwarded-proto,
  x-forwarded-port, x-forwarded-by.

Signed-off-by: Andre Moreira Magalhaes andre.magalhaes@collabora.com

https://phabricator.apertis.org/T9327