Remove unused benchmark fixture in test_dask_mg_random_walks (#4391)

Removes an easy `PytestBenchmarkWarning` from the tests

Authors:
  - Matthew Roeschke (https://github.com/mroeschke)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: #4391

python/cugraph/cugraph/tests/sampling/test_random_walks_mg.py

@@ -202,7 +202,7 @@ def input_graph(request):
 
 @pytest.mark.mg
 @pytest.mark.cugraph_ops
-def test_dask_mg_random_walks(dask_client, benchmark, input_graph):
+def test_dask_mg_random_walks(dask_client, input_graph):
  path_data, seeds, max_depth = calc_random_walks(input_graph)
  df_G = input_graph.input_df.compute().reset_index(drop=True)
  check_random_walks(input_graph, path_data, seeds, max_depth, df_G)