slideshow: that define causes -Wunused-macros with 10.6 SDK too

So since that is the new baseline let's get rid of it.

Change-Id: I20e921afca5e66f22d5403f383fb9fd5f0591142

slideshow/source/engine/OGLTrans/mac/OGLTrans_TransitionerImpl.mm

@@ -28,9 +28,6 @@
 
 #include "sal/config.h"
 
-#if MACOSX_SDK_VERSION < 1070
-#define GLX_GLXEXT_PROTOTYPES 1
-#endif
 #include "OGLTrans_TransitionImpl.hxx"
 
 #include <com/sun/star/beans/XFastPropertySet.hpp>