[TASK] Remove StyleGenerationHook in t3skin

This hook is called inside t3skin and not used anymore because
styling is now done in a better way.
Also classes like bgColor5 are not used anymore.

Resolves: #62797
Releases: master
Change-Id: Idc6e18030c8d96daf1577f51640252fe0ce14d4e
Reviewed-on: http://review.typo3.org/33924
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>