[DistDGL] Fixing warning in test_new_kvstore.

[drivanov]

Description

The following warning

../../../usr/local/lib/python3.10/dist-packages/dgl/heterograph.py:92
  /usr/local/lib/python3.10/dist-packages/dgl/heterograph.py:92: DGLWarning: Recommend creating graphs by `dgl.graph(data)` instead of `dgl.DGLGraph(data)`.
    dgl_warning(

has been fixed. Minor cleanup was done by removing unused import statements.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

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