[JS] Remove superfluous child assignment

[asfimport]

Per PR

Reporter: Wes McKinney / @wesm
Assignee: Adam M Krebs / @akre54

PRs and other links:

• GitHub Pull Request #5394

_{Note: This issue was originally created as ARROW-6785. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 5394
#5394