[KCMultiDialog] Remove most special margins handling; it's done in KPageDialog now

Summary:
See D25444

Depends on D25444

Test Plan: See test plan in D25444

Reviewers: Frameworks, mart, cfeck

Reviewed By: mart

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25451