Remove actioncable server from docker-compose

The actioncable service was added in !444 (merged) and introduced the ability to run a standalone actioncable Puma server in CNG for handling WebSocket requests.

Since then the option to run Action Cable in embedded mode (i.e. bundled in the same server as the rest of the Rails application) has been added. This MR no longer makes standalone mode the default, instead running Action Cable inside the webservice container.

Helm charts issue: gitlab-org/charts/gitlab#2284 (closed)