fix: an issue where changing the transcript language code

[DmytroAlipov]

Description

Fixed an issue where changing the transcript language code would cause both the old and new transcript to be displayed. But in this case, you won’t be able to download the transcript from the old code, since the link is invalid.

The current MR is a consequence of this MR.

To reproduce, do the following:

1. The Course Authoring MFE must be configured, and the new_core_editors.use_new_video_editor flag must be enabled in the admin panel.
2. Add a video to the Unit.
3. Go to the editor. Add a valid transcript.
4. Now you can change the language in an existing transcript.
5. Now, when changing the language, the old one will remain in place and a new one will be added. You will see this when you reload the page.

6. Also, make sure you download the transcripts to your computer. It will only work for the last selected language. For other x there will be an error.

[openedx-webhooks]

Thanks for the pull request, @DmytroAlipov!

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[dyudyunov]

you have the same check in the add_or_update_transcripts function, why do you duplicate it here?

[DmytroAlipov]

@dyudyunov Right, thanks for noticing this redundant code. Initially, I wanted to do it through a separate function. But then I realized that it was redundant. And after moving the logic to studio_transcript I forgot to delete add_or_update_transcripts.

[DmytroAlipov]

I removed the add_or_update_transcripts function.

[idegtiarov]

LGTM!