Marks Mac_mokey microbenchmarks to be unflaky (#181276)

The issue https://github.com/flutter/flutter/issues/174101 has been
closed, and the test has been passing for [50 consecutive
runs](https://data.corp.google.com/sites/flutter_infra_metrics_datasite/flutter_check_test_flakiness_status_dashboard/?p=BUILDER_NAME:%22Mac_mokey%20microbenchmarks%22).
This test can be marked as unflaky.

https://github.com/flutter/flutter/pull/175488 should have been landed.

.ci.yaml

diff --git a/.ci.yaml b/.ci.yaml
index 73855c6..747409a 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -4812,7 +4812,6 @@
 
   # mac mokey benchmark
   - name: Mac_mokey microbenchmarks
-    bringup: true # Flaky https://github.com/flutter/flutter/issues/174101
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60