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.

postorius/Dockerfile.dev

@@ -22,8 +22,8 @@ RUN --mount=type=cache,target=/root/.cache \
 		python3 py3-pip libffi gettext py-cryptography \
     && python3 -m pip install -U pip setuptools wheel \
     && python3 -m pip install -U \
-        git+https://gitlab.com/mailman/mailmanclient@${CLIENT_REF} \
-        git+https://gitlab.com/mailman/postorius@${POSTORIUS_REF} \
+        git+https://gitlab.com/mailman/mailmanclient \
+        git+https://gitlab.com/mailman/postorius \
 		uwsgi \
 		'psycopg2<2.9' \
 		dj-database-url \