Get rid of MATERIAL_MAX_TEXTURES_USED

Another configurable knob of questionable usefulness,
especially since this inhibits certain optimizations
by making loop length dependent on a global variable.

source/Irrlicht/Irrlicht.cpp

@@ -132,7 +132,6 @@ namespace core
 namespace video
 {
 	SMaterial IdentityMaterial;
-	u32 MATERIAL_MAX_TEXTURES_USED = MATERIAL_MAX_TEXTURES;
 }
 
 } // end namespace irr