Remove LICENSE.TXT file

Please review the patch and !6 MR.

Application checklist → is a documentation page on opting into security advisory coverage; it should have not been used to tell maintainers they need to remove the license file.
Drupal Git Contributor Agreement & Repository Usage Policy → does not forbid to add a license file in a project repository; it just says that projects hosted on drupal.org are licensed under the same license used by Drupal core. As long as the license is correct, the file can stay.