Always use angleDelta() for scrolling

Using pixelDelta() resulted in seemingly meaningless values on
KWin/Wayland. Qt code uses angleDelta() exclusively, so I see no reason
not to do the same.

Closes #2236.

client/mapview.cpp

@@ -373,10 +373,7 @@ void map_view::resizeEvent(QResizeEvent *event)
  */
 void map_view::wheelEvent(QWheelEvent *event)
 {
-  auto delta = event->pixelDelta();
-  if (delta.isNull()) {
-    delta = event->angleDelta();
-  }
+  auto delta = event->angleDelta();
 
   if (event->modifiers() == Qt::NoModifier) {
     // Scrolling