[TASK] Remove duplicate css for the btn class

This removes all parts in the button component which are exactly the
same as in bootstrap.

Resolves: #94046
Releases: master
Change-Id: Ie3efb386a4e2cce970fc46155c6ae3b8b9acc82a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68979


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>