[TASK] Remove compat layer for about module

Patch deletes compat layer for class CustomSectionsInterface as it's newly
introduced in TYPO3 6.0

Change-Id: I22b9ffa117099d0ca86b0e8fb50a8d52c6182b8b
Resolves: #40360
Related: #40095
Related: #40240
Releases: 6.0
Reviewed-on: http://review.typo3.org/14173
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik