docker-compose: Remove depends_on

depends_on doesn't make much sense here as it does nothing about
the readiness of containers.

This allows "docker-compose up node" launch the agent without the server.
(Similarly to what docker-compose-agent.yml does)

docker-compose.yml

@@ -20,8 +20,6 @@ services:
     - /run
     - /var/run
     privileged: true
-    depends_on:
-    - server
     environment:
     - K3S_URL=https://server:6443
     - K3S_CLUSTER_SECRET=somethingtotallyrandom