Restore previous tabbar look

Summary:
New tabbar implementation (accidentally?) added small margin between
terminal and tabbar's bottom separating line. This patch removes the
margin.

Comparison:
{F6478598}

Depends on D17605

Test Plan: Tested with default style and custom stylesheet

Reviewers: #konsole, #vdg, tcanabrava, ndavis, hindenburg

Reviewed By: #konsole, #vdg, tcanabrava, ndavis, hindenburg

Subscribers: hindenburg, loh.tar, ngraham, ndavis, abetts, tcanabrava, aaronhoneycutt, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17607