Stop installing apt packages for compiling Python

Now that we've stopped compiling Python from source and instead
download/copy a pre-compiled version, there's no need to install these
apt packages.

A few may be needed for users to build python deps written in native C,
but those shouldn't be listed in this section, they should be added
later in the the Dockerfile where we already list other packages needed
for that:
https://github.com/dependabot/dependabot-core/blob/51b268a81a983c6547af4d95768218a03bd8e751/python/Dockerfile#L128-L143

Also, it's unclear which of these would be needed, so let's start by
removing these, and then as needed can add back the couple that we
actually discover are needed.

python/Dockerfile

@@ -14,30 +14,8 @@ ARG PY_3_8
 ARG PYENV_VERSION
 USER root
 
-# Install *only* the apt packages required for this builder image to build Python.
-# C-libs needed by users to build their Python packages should be installed down below in the final docker image.
-# TODO: not all these packages may be needed now that we've switched from `pyenv install` which compiled from source to
-# downloading / copying pre-compiled python
-RUN apt-get update \
-  && apt-get upgrade -y \
-  && apt-get install -y --no-install-recommends \
-    build-essential \
-    libbz2-dev \
-    libffi-dev \
-    liblzma-dev \
-    libncursesw5-dev \
-    libreadline-dev \
-    libsqlite3-dev \
-    libssl-dev \
-    libxml2-dev \
-    libxmlsec1-dev \
-    tk-dev \
-    xz-utils \
-    zlib1g-dev \
-  && rm -rf /var/lib/apt/lists/*
-
 COPY --chown=dependabot:dependabot python/helpers /opt/python/helpers
-USER root
+
 # TODO: Now that switched from `pyenv install` which compiled from source to downloading / copying a pre-compiled python
 # we could entirely drop pyenv if we change our ruby code that calls `pyenv exec` to track which version of python to
 # call and uses the full python paths.