[TASK] Drop unused scss variables

The following variables are not used or do not differ from their equivalents in bootstrap 5.x:

$font-size-h1
$font-size-h2
$font-size-h3
$font-size-h4
$border-radius-base
$border-radius-large
$border-radius-small

Resolves: #97808
Releases: main
Change-Id: Id783dfe9bc40b7ec1dd88835931409a7dfca9cb8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74979


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