[maintenance] remove local copy of sklearn's check_classification_targets in onedal.utils.validation

[icfaust]

Description

Deletes _check_classification_targets and _type_of_target in onedal.utils.validation.

Only used in one spot (kNNClassifier), is a snapshot of a specific sklearn version, provides no major perf improvement (from first glance), and lacks array API support.

Follow up: address numpy gating in the kNNClassifier's use of it (has some form of array API support on sklearn side since a while).

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Checklist:

Completeness and readability

• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation to reflect the changes or created a separate PR with updates and provided its number in the description, if necessary.
• Git commit message contains an appropriate signed-off-by string (see CONTRIBUTING.md for details).
• I have resolved any merge conflicts that might occur with the base branch.

Testing

• I have run it locally and tested the changes extensively.
• All CI jobs are green or I have provided justification why they aren't.
• I have extended testing suite if new functionality was introduced in this PR.

Performance

• I have measured performance for affected algorithms using scikit-learn_bench and provided at least a summary table with measured data, if performance change is expected.
• I have provided justification why performance and/or quality metrics have changed or why changes are not expected.
• I have extended the benchmarking suite and provided a corresponding scikit-learn_bench PR if new measurable functionality was introduced in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
azure	77.73% <100.00%> (+0.09%)	⬆️
github	71.77% <100.00%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
onedal/utils/validation.py	55.08% <ø> (+0.30%)	⬆️
sklearnex/neighbors/knn_classification.py	84.76% <100.00%> (ø)

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