[TASK] Remove tabs_images for module groups

Backend Modules are mostly recognized by their icons. Those
modules are accessed via the module menu on the left.
Modules are grouped into module groups.

Since module groups are defined the same way as
sub-modules are - also module *groups* have icons.

Those icons are not displayed widely and not maintained.
Because these module -group- icons do not bring benefits:

This patch removes tabs_images from module groups.

Change-Id: I1a1de5d1fa8899c362dcf1d2c592ec85cf32e59b
Resolves: #48774
Releases: 6.2
Reviewed-on: https://review.typo3.org/21193
Reviewed-by: Soren Malling
Tested-by: Soren Malling
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Felix Kopp
Tested-by: Felix Kopp
[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
Merge remote-tracking branch 'origin/master' into 7elix-master