Remove 'bringup: true' from 'Mac mac_unopt' (#50865)

Second half of flutter/flutter#142010

Author: zanderso

.ci.yaml

@@ -387,7 +387,6 @@ targets:
       - os=Mac-13
 
   - name: Mac mac_unopt
-    bringup: true
     recipe: engine_v2/engine_v2
     properties:
       config_name: mac_unopt