[TASK] Clean-up EXT: t3skin css files

Few styles are unused or defined multiple times. Few files are not
in use any longer. Also comments and newlines could be unified.

Change-Id: If38a33291454e5b53247d79b78d2c7141f768512
Resolves: #48776
Releases: 6.2
Reviewed-on: https://review.typo3.org/21194
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp