Cleanup vendoring packages

[liwenwu-amazon]

Right now, github.com/aws/amazon-vpc-cni-k8s uses some code from github.com/aws/amazon-ecs-agent and github.com/aws/amazon-ecs-cni-plugin. These shared code needs to be put into a new repo. And github.com/aws/amazon-vpc-cni-k8s1should vendor these shared code from new repo

[samuelkarp]

What code are you using? It's worth evaluating whether to factor it out or to just stop using the code; not everything in the ECS agent is code that we'd recommend reusing.