chore: remove unused `.eslintignore` and test fixtures (#20316) #3787

Job	Run time
Types (neostandard)	57s
Types (@eslint/json)	59s
Are the types wrong? (eslint-config-eslint, packages/eslint-config-eslint)	17s
Types (eslint-visitor-keys)	1m 15s
Are the types wrong? (eslint, .)	47s
Types (eslint-flat-config-utils)	1m 15s
Types (eslint-webpack-plugin)	0s
	5m 30s

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