remove unused imports in XGBoost notebooks, fix other small style things

[jameslamb]

Contributes to #333

Fixes the following warnings by ruff

source/examples/rapids-1brc-single-node/notebook.ipynb:cell 7:1:1: I001 [*] Import block is un-sorted or un-formatted
source/examples/rapids-1brc-single-node/notebook.ipynb:cell 10:8:17: F541 [*] f-string without any placeholders
source/examples/time-series-forecasting-with-hpo/notebook.ipynb:cell 23:18:9: UP032 [*] Use f-string instead of `format` call
source/examples/time-series-forecasting-with-hpo/notebook.ipynb:cell 103:12:26: F541 [*] f-string without any placeholders
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 3:1:1: I001 [*] Import block is un-sorted or un-formatted
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 3:4:8: F401 [*] `cuml` imported but unused
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 3:10:38: F401 [*] `dask.distributed.wait` imported but unused
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 3:13:21: F401 [*] `sklearn.datasets` imported but unused
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 3:21:8: F401 [*] `gzip` imported but unused
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 11:1:8: F811 [*] Redefinition of unused `time` from cell 3, line 1
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 19:27:9: UP032 [*] Use f-string instead of `format` call
source/examples/xgboost-randomforest-gpu-hpo-dask/notebook.ipynb:cell 20:8:11: UP032 [*] Use f-string instead of `format` call
source/examples/xgboost-dask-databricks/notebook.ipynb:cell 4:1:1: I001 [*] Import block is un-sorted or un-formatted
source/examples/xgboost-gpu-hpo-job-parallel-k8s/notebook.ipynb:cell 20:1:1: I001 [*] Import block is un-sorted or un-formatted
source/examples/xgboost-gpu-hpo-job-parallel-ngc/notebook.ipynb:cell 14:1:1: I001 [*] Import block is un-sorted or un-formatted

How I tested this

Wherever imports were removed, searched through the notebook to be sure they weren't referenced in commented-out code or code in markdown blocks.

[review-notebook-app[bot]]

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