fix: remove incorrect key type when filtering inaccessible blocks

[Agrendalath]

Description

TODO: improve description.

Returning the list with a 'completion' string was added in 9bc0f85. However, the get_accessible_sequences method expects a list of dicts with an 'id' key.

Testing instructions

1. Check that no tests are broken after this change.

Private-ref: BB-8954

[openedx-webhooks]

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[mphilbrick211]

Hi @Agrendalath! Are you still planning to pursue this pull request?

[Agrendalath]

@mphilbrick211, thanks for checking. I'll get back to this PR within the next two weeks.