[R] Various minor cleanups

[asfimport]

Removes some detritus following ARROW-6743 and ARROW-7067, as well as a few other things that have come up.

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

PRs and other links:

• GitHub Pull Request #5796

_{Note: This issue was originally created as ARROW-7103. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 5796
#5796