When tabBarUseUserStyleSheet() is false, don't set a stylesheet at all

Reading the upstream code, it looks like setStyleSheet(QString())[1] will
unset any custom style sheet. This seems like a cleaner solution because
setting a stylesheet at all interferes with changing the widget colorscheme
from a dark to a light scheme or vice versa.

To test:
- open konsole
- open systemsettings and change the colorscheme to a dark one, if you're
  already using a dark one, change to a light one
- widget colors are changed as expected, except for konsole tabs, which look
  odd

Opening a new konsole window seems to clear the issue and the tabs look
normal again.

[1] https://code.woboq.org/qt5/qtbase/src/widgets/kernel/qwidget.cpp.html#_ZN7QWidget13setStyleSheetERK7QString