Revert "BLD-642: Allow multiple transcripts with video." (BLD-899)

@nedbat @cahrens @singingwolfboy @cpennington 

Here's a first pass at reverting https://github.com/edx/edx-platform/commit/25407ef3e73ab947114256c53d2e6ff264c99dd0

I'm not sure I resolved the revert conflicts correctly, would like a review.