Removed unused composeFiles from client.go

[krobertson]

This PR removes the composeFiles variable from compose() and its use in all the calling functions.

I liked how the CLI used a directory structure for instrumenting the docker-compose files, and knew that Compose itself didn't support that, so was curious what it was doing under the hood. In looking over the code, I saw that the composeFiles variable was never used, so figured I'd remove it and clean it up.