Remove tune_cnn.ipynb; fix broken link (#1980)

Summary:
The material in `tune_cnn.ipynb` is redundant with the material in `tune_cnn_service.ipynb`. It is also creating a link because the `tutorials.json` markdown file points to the latter tutorial, while `tutorials/index.js` points to the former. This PR removes `tune_cnn.ipynb` and fixes the broken link by having both point to `tune_cnn_service.ipynb`.

I also removed an unused variable in `index.js`.

Pull Request resolved: #1980

Test Plan: I built tutorials tutorials locally and served the website with ` ./scripts/make_docs.sh` and confirmed that the link works now.

Reviewed By: saitcakmak

Differential Revision: D51207603

Pulled By: esantorella

fbshipit-source-id: da9360cf9feb3d4d8c489e6eb15f8fae5f86eef7