double include of windows

Former-commit-id: e8132ca6086211f7ae061f246afa7ca988cb5d32

include/igl/viewer/OpenGL_shader.h

@@ -29,9 +29,6 @@
 #   include <OpenGL/gl3.h>
 #   define __gl_h_ /* Prevent inclusion of the old gl.h */
 #else
-#   ifdef _WIN32
-#       include <windows.h>
-#   endif
 #   include <GL/gl.h>
 #endif