Remove the 404 HttpCode from 2019-fargate-api.yaml

[MikeTheCanuck]

Currently, to continue testing the 2019 Sandbox API container image in ECS deployment, we've had to enable the ALB Health Check to consider a "404" HTTP response as a healthy container response.

Once we've figured out what's wrong here (container, CloudFormation, whatever it turns out to be) we will remove this as a "healthy" container response, so that incremental deploys of container image builds don't replace a working container with a broken one.