[infra] Remove mac SDK builder test matrix entry.
[infra] Remove mac SDK builder test matrix entry.

Both the production and try builders for the macOS SDK have been
migrated to the new recipe that doesn't use the test matrix.

Bug: b/236109661
Change-Id: I4f578a162c866da6673b228023adb5966f1ca505
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259423
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Jonas Termansen <092ec58085ae5e368bc39d18752740ff81a2d09c@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk