WIP: [ci] [python-package] remove unused mypy type-ignore comments

[jameslamb]

Contributes to #3867.

As more of the library's code gets type annotations, as errors in those annotations are fixed, and as mypy itself is improved, some # type: ignore comments used to suppress mypy errors can become irrelevant.

This PR proposes running mypy with --warn-unused-ignores, which will cause mypy to fail if unnecessary # type: ignore comments are used.

This is useful because # type: ignore comments can sometimes cause mypy to stop checking down a code path, and removing unnecessary ones therefore improves the likelihood that problems can be detected by mypy.

This PR also fixes the following errors introduced by that check.

python-package/lightgbm/basic.py:215: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1056: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1057: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1150: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1151: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1212: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1308: error: Unused "type: ignore" comment
python-package/lightgbm/sklearn.py:1309: error: Unused "type: ignore" comment
python-package/lightgbm/dask.py:1092: error: Unused "type: ignore" comment

[jameslamb]

Going to close this for now to focus on other things. Sorry for the noise.

[github-actions]

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