X-Forwarded-For header should not be added

[julien4215]

The X-Forwarded-For is set to 127.0.0.1 by the plugin.

This is no need to add the X-Forwarded-For header as it is optional according to the event api documentation (https://plausible.io/docs/events-api#request-headers).

It also create a CORS error when a POST request is sent to the event api from a flutter web app. The navigator will first send a preflight request and plausible will not respond with X-Forwarded-For in Access-Control-Allow-Headers resulting in a CORS error. Plausible not adding X-Forwarded-For to Access-Control-Allow-Headers makes sense as the X-Forwarded-For is set by the reverse proxy and should not be set on client side.