Eliminate unnecessary bottom padding on OverlaySheets

Summary:
The OverlaySheet already incorporates topMargin and bottomMargin into its height calculation, so we don't need to add any extra padding.

BUG: 390032

Test Plan:
Tested with Discover's review input sheet.

Before:

After:

Also tested in Kirigami Gallery; all pop-ups I could find still looked good.

Reviewers: Kirigami, apol, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10502