[R] Simplify RTask stop handling

[asfimport]

See #9615 (comment)

Reporter: Neal Richardson / @nealrichardson
Assignee: Romain Francois / @romainfrancois

Related issues:

• [R] use RTasks to manage parallel in converting arrow to R (Is contained by)

PRs and other links:

• GitHub Pull Request #10444

_{Note: This issue was originally created as ARROW-12939. Please see the migration documentation for further details.}