FrameSvg: Simplify updateSizes

Summary:

if (hintXXXMargin > -1) {
    // ...
} else {
    // ...
}

are redundant because fixedXXXMargin already hold correct margin values.

Test Plan: Manually

Reviewers: Plasma, Frameworks, mart

Reviewed By: Plasma, mart

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13402