Remove legacy docker integration

[moskyb]

Within the agent, we have a system whereby the bootstrap script can run things in docker and docker-compose by adding BUILDKITE_DOCKER_* and BUILDKITE_DOCKER_COMPOSE_* environment variables. It's deprecated, and seems like kind of a pain to use.

This is not great! It's significantly less nice to use than our docker and docker-compose plugins.

When we release Agent v4, we should remove this functionality entirely.