remove redundant notebook (rapidsai#668)

closes rapidsai#656

We are retaining `notebooks/Using_Cache.ipynb` which covers use of the image cache. This top-level notebook is not needed.

Authors:
  - Gregory Lee (https://github.com/grlee77)

Approvers:
  - https://github.com/jakirkham
  - Gigon Bae (https://github.com/gigony)

URL: rapidsai#668