Revert "[GitHub][docker] Add python3 venv package to CI container (llvm#161024)"

This reverts commit 5a13c85.

Now that pr-code-format.yml will be using a purpose-built container, we
no longer need support for this in the main CI container.

Author: boomanaiden154

.github/workflows/containers/github-action-ci/Dockerfile

@@ -62,7 +62,6 @@ RUN apt-get update && \
     # Having a symlink from python to python3 enables code sharing between
     # the Linux and Windows pipelines.
     python3-pip \
-    python3-venv \
     file \
     tzdata \
     python-is-python3 && \