Remove FF_USE_LEGACY_VOLUMES_MOUNTING_ORDER feature flag

Overview

In !1352 (merged) we fixed a small regression where the user-defined volumes for the Docker executor are added to the service container. This caused some breaking changes #4260 (closed). We added FF_USE_LEGACY_VOLUMES_MOUNTING_ORDER which allowed the user to use the old behavior where the volume is not mounted to the service container.

Proposal

Remove FF_USE_LEGACY_VOLUMES_MOUNTING_ORDER and have the volume be mounted to the service all the time (which is the default behavior right now)