[TASK] CSS: Remove horizontal gray lines "uppercase"

Class "uppercase" had been used to create visual hierarchy in backend
modules. Now we have H1-H6.

Resolves: #37925
Releases: master
Change-Id: I10eda8bb8a2d49f66c4d587e4cb35fc51be58712
Reviewed-on: http://review.typo3.org/32979
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>