[ci] [python-package] remove unused flake8 configuration

[jameslamb]

As of #5871, this project no longer uses the flake8 library. It uses ruff to enforce similar checks.

This PR proposes removing now-unnecessary configuration for flake8 in setup.cfg.

ruff doesn't rely on the [flake8] table there, and instead gets its configuration from pyproject.toml.

LightGBM/.pre-commit-config.yaml

Lines 21 to 22 in cb4972e

	- id: ruff
	args: ["--config", "python-package/pyproject.toml"]

LightGBM/python-package/pyproject.toml

Line 123 in cb4972e

[tool.ruff.lint]