[BUGFIX] Don't add class to relation fieldset

Resolves: #58482
Releases: 6.2

Change-Id: I2a91c9261124553a4c85497c38fdb5f309f84255
Reviewed-on: https://review.typo3.org/29858
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen