Remove empty `secure_download.sh` (#3071)
Remove empty `secure_download.sh` (#3071)

Summary:
This was never meant to exist; it was introduced accidentally in #3054.

Test Plan:
Running `git grep secure_download` shows no results.

wchargin-branch: rm-secure-download
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard