Remove ECS_UPDATE_DOWNLOAD_DIR (#2373)
Remove ECS_UPDATE_DOWNLOAD_DIR (#2373)

we are removing this env var from our docs because it was never intended
to be a publically-configurable variable.

for this env var to function it needs to be integrated with ecs-init and
    the directory needs to be mounted in docker, so it has never
    functioned the way customers would expect.
aws/amazon-ecs-agent,swipely/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-agent,aws/amazon-ecs-agent,swipely/amazon-ecs-agent,swipely/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-agent,swipely/amazon-ecs-agent