Remove the extra installation of mac os in the engine recipe.

This is to prepare the builders to use the new mac capacity with mac
10.15.

Change-Id: I7fc4c7f1eaa82b5e9002fab0c9290bf5c5f89efe
Bug: https://github.com/flutter/flutter/issues/65129
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/6300
Reviewed-by: Christopher Fujino <fujino@google.com>
Commit-Queue: Godofredo Contreras <godofredoc@google.com>