LibGUI: Don't hard-code TreeView row height

Instead of overriding AbstractTableView::row_height() and returning a
hard-coded height for some reason, just remove the override.

This makes tree view row heights honor the current font size.

Userland/Libraries/LibGUI/TreeView.h

@@ -55,7 +55,6 @@ protected:
 private:
     virtual ModelIndex index_at_event_position(Gfx::IntPoint, bool& is_toggle) const override;
 
-    int row_height() const { return 16; }
     int max_item_width() const { return frame_inner_rect().width(); }
     int indent_width_in_pixels() const { return 16; }
     int icon_size() const { return 16; }