Marks Mac_arm64_mokey entrypoint_dart_registrant unflaky (#181648)

There's only been one flake in the last 50 runs and it was due to NDK
download timeouts, which is fixed
https://github.com/flutter/flutter/pull/179647.

https://ci.chromium.org/ui/p/flutter/builders/prod/Mac_arm64_mokey%20entrypoint_dart_registrant/7047/overview

.ci.yaml

diff --git a/.ci.yaml b/.ci.yaml
index 9777fb0..06cdeee 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -4694,7 +4694,6 @@
 
   # mac mokey test
   - name: Mac_arm64_mokey entrypoint_dart_registrant
-    bringup: true # Flakey https://github.com/flutter/flutter/issues/179514
     recipe: devicelab/devicelab_drone
     presubmit: false
     timeout: 60