[C++] Avoid needless c-ares detection #28127
Description
If we use system gRPC, we don't need to detect c-ares explicitly.
Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou
PRs and other links:
Note: This issue was originally created as ARROW-12326. Please see the migration documentation for further details.