remove unused imports

[jameslamb]

Contributes to #6403

While working on that, my IDE highlighted a few unused imports. Looked into that a bit and saw that this project is currently configured to ignore flake8's warnings about unused imports:

cuml/python/cuml/.flake8

Lines 27 to 28 in c5c0656

	# imported but unused
	F401

I went back to the PR that introduced that (#4983) and didn't see any discussion that made me think that was the intended long-term state.

This proposes:

• enforcing flake8's unused imports check
• removing unused imports

Notes for Reviewers

Benefits of these changes

• faster imports
• reduced exposure to changes in dependencies
• reduced effort required to identify and remove dead code
• reduced risk of import issue in alternative builds of the package (like [BUG] cuml-cpu 25.04 fails to import without a GPU present #6403)

Approach I took

Removed F401 exclusion from .flake8, then ran

pre-commit run --all-files

Manually went through the suggestions and looked at the usage.

For library code:

• added # noqa: F401 for imports that looked like they were intentional (e.g. for their side effects)
• removed anything that looked unused

For test code:

• preserved any unused imports from cuml itself, via #noqa comments
  • just importing from a module gives you some minimal test coverage of it, I didn't want to accidentally remove any of that
• removed anything else that looked unused

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[jameslamb]

/ok to test

[csadorf]

Fantastic!

[csadorf]

/merge

[jcrist]

Thanks James!