do not install cudf in cudf_polars wheel tests (#16612)

Removes unnecessary installation of `cudf` wheels in wheel testing for `cudf_polars`.

`cudf_polars` doesn't depend on `cudf`, and neither do its tests. However, right now it's downloading `cudf` during it's wheel tests. I mistakenly introduced that in #16575.

This introduced a race condition that could lead to CI failures whenever the `cudf` wheels aren't published yet by the time the `cudf_polars` tests. Because the `cudf_polars` wheel tests (rightly) do not wait for `cudf` wheels to be available:

https://github.com/rapidsai/cudf/blob/555734dee7a8fb10f50c8609a8e4fb2c025e6305/.github/workflows/pr.yaml#L154-L155

https://github.com/rapidsai/cudf/blob/555734dee7a8fb10f50c8609a8e4fb2c025e6305/.github/workflows/pr.yaml#L145-L146

Noticed this in #16611

```text
[rapids-download-from-s3] Downloading and decompressing s3://rapids-downloads/ci/cudf/pull-request/16611/a6b7eff/cudf_wheel_python_cudf_cu12_py310_x86_64.tar.gz into ./dist
download failed: s3://rapids-downloads/ci/cudf/pull-request/16611/a6b7eff/cudf_wheel_python_cudf_cu12_py310_x86_64.tar.gz to - An error occurred (404) when calling the HeadObject operation: Not Found
```

([build link](https://github.com/rapidsai/cudf/actions/runs/10472939821/job/29004728278?pr=16611))

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #16612

ci/test_wheel_cudf_polars.sh

@@ -20,15 +20,13 @@ fi
 RAPIDS_PY_CUDA_SUFFIX="$(rapids-wheel-ctk-name-gen ${RAPIDS_CUDA_VERSION})"
 RAPIDS_PY_WHEEL_NAME="cudf_polars_${RAPIDS_PY_CUDA_SUFFIX}" RAPIDS_PY_WHEEL_PURE="1" rapids-download-wheels-from-s3 ./dist
 
-# Download the cudf and pylibcudf built in the previous step
-RAPIDS_PY_WHEEL_NAME="cudf_${RAPIDS_PY_CUDA_SUFFIX}" rapids-download-wheels-from-s3 ./dist
+# Download pylibcudf built in the previous step
 RAPIDS_PY_WHEEL_NAME="pylibcudf_${RAPIDS_PY_CUDA_SUFFIX}" rapids-download-wheels-from-s3 ./dist
 
 rapids-logger "Installing cudf_polars and its dependencies"
 
 # echo to expand wildcard before adding `[extra]` requires for pip
 python -m pip install \
-    "$(echo ./dist/cudf_${RAPIDS_PY_CUDA_SUFFIX}*.whl)" \
     "$(echo ./dist/cudf_polars_${RAPIDS_PY_CUDA_SUFFIX}*.whl)[test]" \
     "$(echo ./dist/pylibcudf_${RAPIDS_PY_CUDA_SUFFIX}*.whl)"