Revert "[BUGFIX] Button typography"

This reverts commit 590f76cc4b3bc86ee06fafc1a2a30f90d5b1d56c

This is not in line with the rest of the backend yet.
The "UX vision" is for after 6.2.

Change-Id: Iabb64ac3a278a3a3e1423283a9267aa5996343f1
Reviewed-on: https://review.typo3.org/27307
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny