ARROW-13358 added support for factors in the if_else() and ifelse() functions but we did not remove the warning message saying that factors are currently converted to strings (which is no longer true). Remove the warning and modify the test that has a comment referring to ARROW-13358.

Reporter: Ian Cook / @ianmcook
Assignee: Stephanie Hazlitt / @stephhazlitt

Related issues:

• [R] Include unused factor levels in coalesce() and if_else() output (is related to)
• [C++] Extend type support for if_else kernel (depends upon)

PRs and other links:

• GitHub Pull Request #11794

_{Note: This issue was originally created as ARROW-14659. Please see the migration documentation for further details.}