remove some gcc4.8/windows special code as minimum gcc version to build rt is 4.9

rtgui/filebrowserentry.cc

@@ -205,11 +205,7 @@ void FileBrowserEntry::updateImage (rtengine::IImage8* img, double scale, rtengi
         cropParams
     };
 
-#if __GNUC__ == 4 && __GNUC_MINOR__ == 8 && defined( WIN32 ) && defined(__x86_64__)
-    const gint priority = G_PRIORITY_DEFAULT;
-#else
     const gint priority = G_PRIORITY_LOW;
-#endif
 
     const auto func = [](gpointer data) -> gboolean {
         tiupdate* const params = static_cast<tiupdate*>(data);

rtgui/thumbimageupdater.cc

@@ -68,11 +68,8 @@ public:
         inactive_waiting_(false)
     {
         int threadCount = 1;
-#if !(__GNUC__ == 4 && __GNUC_MINOR__ == 8 && defined( WIN32 ) && defined(__x86_64__))
-        // See Issue 2431 for explanation
 #ifdef _OPENMP
         threadCount = omp_get_num_procs();
-#endif
 #endif
 
         threadPool_ = new Glib::ThreadPool(threadCount, 0);