Remove outdated C++ Extensions tutorial, redirect to custom ops (#3547)

As titled. This tutorial is outdated and people should read the custom
ops one instead.

## Checklist
<!--- Make sure to add `x` to all items in the following checklist: -->
- [x] The issue that is being fixed is referred in the description (see
above "Fixes #ISSUE_NUMBER")
- [x] Only one issue is addressed in this pull request
- [x] Labels from the issue that this PR is fixing are added to this
pull request
- [x] No unnecessary issues are included into this pull request.

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