Re-disable microbenchmarks iOS. (flutter#166788)

This went flakey/flaky immeidately

Author: Jonah Williams

.ci.yaml

@@ -5132,6 +5132,7 @@ targets:
   - name: Mac_ios microbenchmarks_ios
     recipe: devicelab/devicelab_drone
     presubmit: false
+    bringup: true
     timeout: 60
     properties:
       tags: >