remove per-fingerprint ACLs that are identical to keyring ACL

With the changes in 707a89a3b86961755a99cb9e1a0a5f23690f9529 dak uses
the keyring acl if there is no acl for the fingerprint. Also
import_keyring.py stopped updating the fingerprint acl. This caused
problems for old fingerprints (with an acl set) that move between
keyrings as they would continue to use the old acl.

This update sets the acl to NULL for fingerprints using the keyring acl
to avoid these issues.