Don't override the source/version of the forum code.

These overrides caused me to run around a bit, trying to figure out why
changing the forums version in the defaults file for the forums playbook
wasn't taking.

If people need to, they can override the source/version in their
respective branches/PRs, but by default, it should do no such default
overriding.

docker/build/forum/ansible_overrides.yml

@@ -4,10 +4,8 @@ FLOCK_TLD: "edx"
 
 FORUM_MONGO_HOSTS:
   - mongo.{{ FLOCK_TLD }}
-  
+
 FORUM_ELASTICSEARCH_HOST: "es.{{ FLOCK_TLD }}"
-forum_source_repo: "https://github.com/open-craft/cs_comments_service.git"
-forum_version: "mongoid5"
 FORUM_USE_TCP: "true"
 FORUM_RACK_ENV: "staging"
-FORUM_SINATRA_ENV: "staging"
\ No newline at end of file
+FORUM_SINATRA_ENV: "staging"