[R] R package namespace cleanup

[asfimport]

Pulling off the cleanup bits from ARROW-5505.

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

PRs and other links:

• GitHub Pull Request #4566

_{Note: This issue was originally created as ARROW-5600. Please see the migration documentation for further details.}

[asfimport]

Romain Francois / @romainfrancois:
Issue resolved by pull request 4566
#4566