Remove unused namespace from ui export

[yuriyDne]

Description

The Export model have some unused namespaces: Magento\Framework\Filesystem\Directory\WriteInterface.
Just removing it and update description

Manual testing scenarios

1. For example, go to orders list;
2. Export list of order by XML or CSV;
3. All should export as expected.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

@yuriyDne thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @yuriyDne. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.