[R] test-r-linux-as-cran nightly fails with NOTE about non-standard top-level file jarl.toml #51152

The test-r-linux-as-cran nightly job has failed on every run since 2026-08-25 (last successful run was 2026-08-21). All five rhub jobs fail the same way:

Found R CMD check NOTE(s) in output:
* checking top-level files ... NOTE
Non-standard file/directory found at top level:
  'jarl.toml'

Example failing run: https://github.com/ursacomputing/crossbow/actions/runs/33577466028/job/100084471107

r/jarl.toml was added in #50851 (71c98fc) but was not added to r/.Rbuildignore, so it ends up in the package tarball and R CMD check flags it. Fix is to add ^jarl\.toml$ to https://github.com/apache/arrow/blob/1b0b16a8f68c0082534aa185c0bb4052614df924/r/.Rbuildignore

That PR also left r/.lintr and its .Rbuildignore entry in place, which can be removed at the same time.