Restore default tabbar style when custom stylesheet is disabled

Test Plan:
* Open new tab to show tabbar
* Enable //Use user-defined stylesheet// in Konsole Settings
* Set some stylesheet file; example contents:
```
QTabBar::tab { background-color: #0f0; border: 0; color: #000; }
```
* Apply settings
* Disable //Use user-defined stylesheet// in Konsole Settings
* Apply settings

**Actual result:**
Custom style is still used by tabbar
**Expected result:**
Default tabbar style is restored

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17605

(cherry picked from commit 5260daa6)