tar mtree module isn't available in all macOS distros.

Disable use of mtree module to strip variant file data
from packages.

Change-Id: I9a4515258f5daf5a87d9b5410b6cd7398ae48970

Author: Stewart Miles

source/ExportUnityPackage/export_unity_package.py

@@ -2507,10 +2507,6 @@ def create_archive(archive_filename, input_directory, timestamp):
                 os.utime(filename, (FLAGS.timestamp, FLAGS.timestamp))
             # Don't recurse directories.
             tar_args.append("-n")
-            # If timestamp, group or owner are present strip all fields from
-            # the tar so that it's reproducible.
-            if FLAGS.timestamp or FLAGS.owner or FLAGS.group:
-              tar_args.extend(["--options", "mtree:!all"])
           tar_args.extend(["-T", list_filename])
           # Disable timestamp in the gzip header.
           tar_env = os.environ.copy()