Revert "Set environment variable QT_AUTO_SCREEN_SCALE_FACTOR in Qt 5.6."

This reverts commit 85e1b4d089. Setting
that environment variable breaks the UI completely for certain
environments. If Qt itself doesn't enable auto scaling by default,
neither should Creator (for good reason, as it apparently  doesn't work
yet on X11).

Worse, Qt Creator must respect the environment settings set by the user,
never override it. This commit was unconditionally setting the auto
scaling, making it impossible for me to override the settings.

Task-number: QTCREATORBUG-15115
Change-Id: I42e7ef1a481840699a8dffff140857f6c0b33713
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/app/main.cpp

@@ -299,8 +299,6 @@ int main(int argc, char **argv)
             && !qEnvironmentVariableIsSet("QT_DEVICE_PIXEL_RATIO")) {
         qputenv("QT_DEVICE_PIXEL_RATIO", "auto");
     }
-#else
-    qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "1");
 #endif // < Qt 5.6
 
     QLoggingCategory::setFilterRules(QLatin1String("qtc.*.debug=false"));