Remove mac prod builder. (#65481)

This is in preparation to enable the new mac builders.

Bug:
  https://github.com/flutter/flutter/issues/65223

dev/prod_builders.json

@@ -66,12 +66,6 @@
          "task_name":"linux_web_smoke_test",
          "flaky":false
       },
-      {
-         "name": "Mac",
-         "repo": "flutter",
-         "task_name": "mac_bot",
-         "flaky": false
-      },
       {
          "name": "Windows build_tests",
          "repo": "flutter",