Hide tabbar in KParts

Summary:
After QTabBar revamp, correct issue where tabs were still
showing in KParts.

Test Plan:
Opened instances of dolphin / kate and looked the kparts output.
Opened konsole, everything seems fine.

Reviewers: hindenburg, #konsole

Reviewed By: hindenburg, #konsole

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D16706