Removed references to ECS from AWS credentials documentation

Author: -- Gordon Shankman

docs/drivers/aws.md

@@ -23,7 +23,6 @@ to authenticate with AWS:
 1.  Command line flags
 1.  Environment variables
 1.  AWS credentials file
-1.  ECS Task Role
 1.  EC2 Instance Role
 
 For more information, see the [AWS SDK for Go Developer's Guide](http://docs.aws.amazon.com/sdk-for-go/v1/developerguide/configuring-sdk.html).
@@ -57,11 +56,6 @@ This is the simplest method. You can then create a new machine with:
 
     $ docker-machine create --driver amazonec2 aws01
 
-### ECS Task Role
-
-When you deploy tasks and services on Amazon's EC2 Container Service (ECS) platform, you can specify an IAM role
-at the container level. The amazonec2 driver will use the injected credentials if they are found.
-
 ### EC2 Instance Role
 
 If you are running docker-machine from an existing EC2 instance, the amazonec2 driver will use the credentials for the IAM role