Only keep one Kirigami theme instance for all users

Fetching the Kirigami Theme via attached properties is expensive. The
theme is also going to be the same for all users so it's enough to only
do it once.

src/neochatuser.h

@@ -32,7 +32,6 @@ Q_SIGNALS:
     void colorChanged(QColor _t1);
 
 private:
-    Kirigami::PlatformTheme *m_theme = nullptr;
     QColor m_color;
 
     void polishColor();