Remove some unneeded horizontal line from MyCentralWidget

The difference is minimal and probably looks very bad with dark
and custom themes.

Let's minimize the amount of hardcoded styling.

src/MainWindow.cpp

@@ -61,14 +61,6 @@ public:
     }
 
     ~MyCentralWidget() override;
-
-    void paintEvent(QPaintEvent *) override
-    {
-        QPainter p(this);
-        QPen pen(QColor(184, 184, 184, 184));
-        p.setPen(pen);
-        p.drawLine(0, 0, width(), 0);
-    }
 };
 }