Remove db_clean service from docker-compose.yml

A race condition between these two containers was causing the database
to sometimes get cleaned after migrations had been run. Rather than hack
together scripts to track this state I'm simply removing the clean
service from the docker-compose configuration.

See also:
  * https://docs.docker.com/compose/startup-order/
  * docker/compose#374

docker-compose.yml

@@ -5,16 +5,6 @@ services:
     image: postgres:11.6
     ports:
       - 5432:5432
-  db_clean:
-    build:
-      context: .
-      dockerfile: Dockerfile.db_clean
-    environment:
-      - FLYWAY_USER=postgres
-      - FLYWAY_PASSWORD=
-      - FLYWAY_URL=jdbc:postgresql://db/postgres
-    depends_on:
-      - db
   db_migrate:
     build:
       context: .
@@ -26,7 +16,7 @@ services:
     volumes:
       - ./migrations:/flyway/sql
     depends_on:
-      - db_clean
+      - db
   easi:
     build:
       context: .