[CLEANUP] Remove useless CSS file in EXT:documentation

Releases: 6.2
Resolves: #50728
Change-Id: I2de2182d094b8e8eb5373c38f57e20331ea3d4d6
Reviewed-on: https://review.typo3.org/22763
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers