WPTV: Subtitle attachment info stays even after subtitle file is removed

[Nao]

When an approved subtitle file is deleted, it remains as approved subtitles. Because of this, a new file for the same language can't be uploaded.

Steps to reproduce:

1. Upload a subtitle file
2. Approve that subtitles
3. Delete the subtitle file (WITHOUT unchecking ”approved”)
4. The video still displays the subtitle language on the frontend sidebar, but if you click the language link you get an error: LOCALE is not a supported language (e.g. see the ​English (Canada) link, attached to ​this video)
5. Also, from the backend of the video's attachment information, the subtitle seems not selected. In the example case above, the English (Canada) subtitle attachment ID value is 0 (​admin link).

Expected behavior:
When an approved subtitle file is deleted, WPTV should consider that the video no longer has subtitles for the language.

[dd32]

Just noting that I've cleaned some up manually:

​https://wordpress.tv/2021/06/03/shusei-toda-block-editor-02-text-blocks/

wp> $subtitles = get_post_meta( 108717, '_videopress_subtitles', true );
=> array(3) {
  ["ja"]=>
  array(2) {
    ["language"]=>
    string(2) "ja"
    ["subtitles_post_id"]=>
    int(108792)
  }
  ["en"]=>
  array(2) {
    ["language"]=>
    string(2) "en"
    ["subtitles_post_id"]=>
    int(109539)
  }
  ["en-ca"]=>
  array(2) {
    ["language"]=>
    string(5) "en-ca"
    ["subtitles_post_id"]=>
    int(109540)
  }
}
wp> unset( $subtitles['en-ca'] );
wp> update_post_meta( 108717, '_videopress_subtitles', $subtitles );
=> bool(true)

​https://wordpress.tv/2021/02/06/muhammad-muhsin-introduction-to-wordpress/

wp> $subtitles = get_post_meta( 107951, '_videopress_subtitles', true );
=> array(1) {
  ["en"]=>
  array(2) {
    ["language"]=>
    string(2) "en"
    ["subtitles_post_id"]=>
    int(107954)
  }
}
wp> unset( $subtitles['en'] );
wp> update_post_meta( 107951, '_videopress_subtitles', $subtitles );
=> bool(true)