Cleanup: remove unused get_screen_dpi() function

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/display.h

@@ -8,8 +8,6 @@
 extern "C" {
 #endif
 
-extern double get_screen_dpi(void);
-
 /* Plot info with smoothing, velocity indication
  * and one-, two- and three-minute minimums and maximums */
 struct plot_info {

subsurface-helper.cpp

@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <QQmlEngine>
-#include <QDesktopWidget>
-#include <QApplication>
 #include <QDebug>
 #include <QQuickItem>
 
@@ -55,13 +53,6 @@ void exit_ui()
 	free((void *)existing_filename);
 }
 
-double get_screen_dpi()
-{
-	QDesktopWidget *mydesk = qApp->desktop();
-	return mydesk->physicalDpiX();
-}
-
-
 void run_ui()
 {