Remove the get_latest_ref script and use the primary branch.

We previously used the get_latest_ref.py script to fetch the
latest commit sha for each project that passed CI correctly. Although,
that has some challenges due to requiring Gitlab Auth token. It causes
issues with building PRs and we don't want to share the Gitlab token
with PR authors.

Instead, this commit removes the code and sha references and Instead
simply uses the primary master branch from each project to build the
rolling release container image.

README.md

@@ -89,36 +89,24 @@ $ docker pull docker.io/maxking/mailman-web:rolling
 $ docker pull docker.io/maxking/mailman-core:rolling
 ```
 
-Rolling releases are built with every commit and also re-generated weekly. You
+Rolling releases are built with every commit and also re-generated nightly. You
 can inspect the images to get which commit it was built using:
 
 ```bash
 $ docker inspect --format '{{json .Config.Labels }}' mailman-core | python -m json.tool
 {
-    "version.core": "31f434d0",
     "version.git_commit": "45a4d7805b2b3d0e7c51679f59682d64ba02f05f",
-    "version.mm3-hk": "c625bfd2"
 }
 
 $ docker inspect --format '{{json .Config.Labels }}' mailman-web | python -m json.tool
 {
-    "version.client": "d9e9cb73",
-    "version.dj-mm3": "72a7d6c4",
     "version.git_commit": "45a4d7805b2b3d0e7c51679f59682d64ba02f05f",
-    "version.hyperkitty": "b67ca8a8",
-    "version.postorius": "73328ad4"
 }
 
 ```
 
 - `version.git_commit` : This is the commit hash of the Dockerfile in the
   [Github repo](https://github.com/maxking/docker-mailman)
-- `version.core`: The commit hash of Mailman Core
-- `version.mm3-hk`: The commit hash of Mailman3-hyperkitty plugin.
-- `version.client`: The commit hash of Mailman Client.
-- `version.hyperkitty`: The commit hash of Hyperkitty.
-- `version.postorius`: The commit hash of Postorius.
-- `version.dj-mm3`: The commit hash of Django-Mailman3 project.
 
 ## Dependencies
 
@@ -414,7 +402,7 @@ smtp_port: $SMTP_PORT
 configuration: /etc/postfix-mailman.cfg
 ```
 
-So, if you need to update the values, you can set `SMTP_HOST`, `SMTP_PORT`, 
+So, if you need to update the values, you can set `SMTP_HOST`, `SMTP_PORT`,
 `MM_HOSTNAME` environment variables in `mailman-core` container.
 
 Please verify the output for `[mta]` section to ensure that it points to