define IGL_HEADER_ONLY should **Never** appear in file

Former-commit-id: 16ef75a5b545feda3333886549aac8d0adee7c22

include/igl/viewer/Viewer.h

@@ -15,7 +15,6 @@
 #define IGL_MOD_SUPER           0x0008
 
 #ifdef ENABLE_XML_SERIALIZATION
-  #define IGL_HEADER_ONLY
   #include <igl/xml/XMLSerializer.h>
   #include <igl/xml/XMLSerialization.h>
 #endif