cli: hide import-sig --write option

It is no more used, hide it from the user.

Fixes: https://pagure.io/koji/issue/2620

tests/test_cli/test_import_sig.py

@@ -276,7 +276,6 @@ class TestImportSIG(utils.CliTestCase):
 Options:
   -h, --help       show this help message and exit
   --with-unsigned  Also import unsigned sig headers
-  --write          Also write the signed copies
   --test           Test mode -- don't actually import
 """ % self.progname)