Remove isExtendingEslintConfig variable on eject

[lukyth]

The part of webpack config that use isExtendingEslintConfig will be removed on eject, the variable itself hasn't been removed yet. This PR makes the variable to be removed on eject as well.

The code block that use this variable:

// @remove-on-eject-begin
ignore: isExtendingEslintConfig,
baseConfig: isExtendingEslintConfig
  ? undefined
  : {
      extends: [require.resolve('eslint-config-react-app')],
    },
useEslintrc: isExtendingEslintConfig,
// @remove-on-eject-end

[mrmckeb]

Hi @lukyth, thanks for the PR. Good news, we're actually removing this feature in the next release.

This will be resolved in #9587 :)

Thanks for your time! This would have been a great candidate had we not planned to remove the flag.