Enables optional access to Etherpad's web-UI. This is useful for
managing Etherpad plugins.
Among other things, plugins makes it easy to manage/delete pads if you
install the adminpads2 plugin.
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)