Revert "Adapted to Magnum changes."

Don't commit without testing.

This reverts commit e709ad800bb7ca4b63ddab4b4e8b3ae1f8113fb7.

src/Plugins/TgaImporter/TgaImporter.cpp

@@ -20,6 +20,7 @@
 #include "Trade/ImageData.h"
 
 using namespace std;
+using namespace Corrade::Utility;
 
 namespace Magnum { namespace Trade { namespace TgaImporter {