sync: dropped "NOTICE: --use-superproject is in beta ..." message.

Tested the code with the following commands.

$ ./run_tests -v

Bug: [google internal] b/209511230
Change-Id: Ia3c6de47709f5276e324a5bb608383aba3b2c562
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/327197
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>
diff --git a/git_superproject.py b/git_superproject.py
index 4ca84a5..c972322 100644
--- a/git_superproject.py
+++ b/git_superproject.py
@@ -226,8 +226,6 @@
                        f'{self._manifest.manifestFile}')
       return SyncResult(False, False)
 
-    print('NOTICE: --use-superproject is in beta; report any issues to the '
-          'address described in `repo version`', file=sys.stderr)
     should_exit = True
     if not self._remote_url:
       self._LogWarning(f'superproject URL is not defined in manifest: '