[R] Remove duplicate setting of S3 flag on macOS

[asfimport]

Observed here: https://github.com/autobrew/scripts/pull/3#issuecomment-712494384

It doesn't hurt anything but might as well clean it up.

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #8502

_{Note: This issue was originally created as ARROW-10365. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 8502
#8502