➖(dimail) remove ci-time dependency on dimail to improve CI times

The Gitlab instance hosting dimail images has become unstable
resulting in excessive CI failures, and we don't actually depend
on dimail for CI.

docker-compose.yml

@@ -36,7 +36,6 @@ services:
       - ./data/media:/data/media
       - ./data/static:/data/static
     depends_on:
-        - dimail
         - postgresql
         - maildev
         - redis