[misc] cleanup unused dependency on mongo

services/real-time/buildscript.txt

@@ -4,7 +4,7 @@ real-time
 --env-add=
 --node-version=10.19.0
 --acceptance-creds=None
---dependencies=mongo,redis
+--dependencies=redis
 --docker-repos=gcr.io/overleaf-ops
 --env-pass-through=
 --script-version=1.3.5

services/real-time/docker-compose.ci.yml

@@ -25,8 +25,6 @@ services:
       MOCHA_GREP: ${MOCHA_GREP}
       NODE_ENV: test
     depends_on:
-      mongo:
-        condition: service_healthy
       redis:
         condition: service_healthy
     user: node
@@ -43,5 +41,3 @@ services:
   redis:
     image: redis
 
-  mongo:
-    image: mongo:3.6

services/real-time/docker-compose.yml

@@ -32,8 +32,6 @@ services:
       NODE_ENV: test
     user: node
     depends_on:
-      mongo:
-        condition: service_healthy
       redis:
         condition: service_healthy
     command: npm run test:acceptance
@@ -41,6 +39,3 @@ services:
   redis:
     image: redis
 
-  mongo:
-    image: mongo:3.6
-