Remove Mac mac_unopt presubmit retry count (#164350)

flutter/flutter#157636 (comment)

fyi: @flar

Author: jtmcdole

engine/src/flutter/.ci.yaml

@@ -487,8 +487,6 @@ targets:
     properties:
       config_name: mac_unopt
       add_recipes_cq: "true"
-      # Retry for flakes caused by https://github.com/flutter/flutter/issues/157636
-      presubmit_max_attempts: "2"
     timeout: 120
 
   - name: Mac mac_ios_engine