Video: Revert Download transcript button behavior.

Reverts the behavior of `Download Transcript` button that was added in https://github.com/edx/edx-platform/pull/2149 .
Ticket: [BLD-811](https://edx-wiki.atlassian.net/browse/BLD-811)
Some code is commented out because might be used in future.

@valera-rozuvan please review.