Remove redundant include statements

Author: DongXu

shell/platform/darwin/ios/ios_external_texture_metal.mm

@@ -8,7 +8,6 @@
 #include "third_party/skia/include/core/SkYUVAIndex.h"
 #include "third_party/skia/include/gpu/GrBackendSurface.h"
 #include "third_party/skia/include/gpu/mtl/GrMtlTypes.h"
-#include "third_party/skia/src/gpu/gl/GrGLDefines.h"
 
 namespace flutter {