[LX-2552]: Fix `annotated_video` and `audio` blocks in pathways

[giovannicimolin]

This merge request fixes two XBlocks that don't work properly on pathways:

1. lx_audio: removed code cruft and dependencies on transcript files (not used anymore).
2. annotated_video: fixed code that locates child video block to work in a generic way.

Jira tickets

• FAL-2925
• LX-2552

Testing instructions

1. Install this branch on your devstack.
2. Check that lx_audio and annotated_videos continue working from the library (http://localhost:4556/library/items/lb:LabXchange:1109ec43:lx_annotated_video:1 and http://localhost:4556/library/items/lb:LabXchange:1f29b8d5:lx_audio:1).
3. Check that the modified blocks retain their behavior when logged out.
4. Create a pathways adding those two items and check that they work.

Author's notes and concerns

1. I also took the opportunity to fix the tests and remove unused bits of code!

Release notes

Release as usual - this will only fix the currently broken blocks.

Reviewers

• [ ] @pomegranited

[pomegranited]

I've got one nit above, but 👍 conditional on resolving that.

Thank you for sorting this out @giovannicimolin !

• [x] I tested this on my devstack using the PR instructions. I also ensured that:
  • Video blocks and their transcripts work as expected, when standalone, in Annotated Videos, and when in Pathways.
  • Audio transcripts work as expected, standalone and in Pathways.
  • Unit tests pass.
• [x] I read through the code
• [x] I checked for accessibility issues N/A
• [x] Includes documentation N/A
• [x] I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository. N/A