X-Forwarded-For header should not be added

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.