Consolidate get_transcript methods

There were two get_transcript methods. The broken one that was being
used (VideoTranscriptsMixin.get_transcript) is stripped out here - it
has been superseded by transcripts_utils.get_transcript. The latter
includes support for blockstore and VAL, while the former did not.

This fixes the `AttributeError: 'LibraryLocatorV2' object has no
attribute 'make_asset_key'` error seen when attempting to load a
transcript from a video through the xblock api when the video had the
transcript stored in blockstore.

Note that if you were previously using video.get_transcript, you should
now use `transcripts_utils.get_transcript(video, ...)`, and note that
the returned 'filename' will be prefixed with the language code, as
other `get_transcript*` functions already do.