gui: Remove QT_VERSION fallbacks for Qt < 5

There were surprisingly many `#ifdef` fallbacks for Qt 4.

Remiving them simplifies maintenance, as well as adding new GUI
functionality.

src/qt/networkstyle.cpp

@@ -68,11 +68,7 @@ NetworkStyle::NetworkStyle(const QString &_appName, const int iconColorHueShift,
         }
 
         //convert back to QPixmap
-#if QT_VERSION >= 0x040700
         pixmap.convertFromImage(img);
-#else
-        pixmap = QPixmap::fromImage(img);
-#endif
     }
 
     appIcon             = QIcon(pixmap);