[TASK] dbal: Remove obsolete CSS HTML template

The HTML template was used in the old install tool, is
unused now and can be removed.

Change-Id: I046c772acd2a9db166036216b3d8ed78ffe813e4
Resolves: #51469
Releases: 6.2
Reviewed-on: https://review.typo3.org/23428
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters