Revert "trust the reverse proxy by default"

This reverts commit fd3d48bb6d.

Normally this environment variable gets referred to from `settings.json.docker`,
but we have our own full configuration, which hardcodes `"trustProxy": true`,
thus making this useless.

This has been pointed out here:
fd3d48bb6d (commitcomment-47403097)

roles/matrix-etherpad/defaults/main.yml

@@ -23,12 +23,7 @@ matrix_etherpad_user_gid: '5001'
 matrix_etherpad_container_http_host_bind_port: ''
 
 # A list of extra arguments to pass to the container
-#
-# We assume that a reverse proxy is used and tell the container to trust it
-#   Details: https://github.com/ether/etherpad-lite/blob/develop/doc/docker.md
-matrix_etherpad_container_extra_arguments: [
-  '--env TRUST_PROXY=true'
-]
+matrix_etherpad_container_extra_arguments: []
 
 matrix_etherpad_public_endpoint: '/etherpad'