Remove outdated C++ Extensions tutorial, redirect to custom ops

Author: janeyx99

advanced_source/README.txt

@@ -8,7 +8,3 @@ Advanced Tutorials
 2. numpy_extensions_tutorial.py
 	Creating Extensions Using numpy and scipy
 	https://pytorch.org/tutorials/advanced/numpy_extensions_tutorial.html
-
-3. cpp_extension.rst
-	Custom C Extensions for PyTorch
-	https://docs.pytorch.org/tutorials/advanced/cpp_extension.html