dsdb/encrypted_secrets: remove dependency to libnettle and use our own aes_gcm_128_*()

dsdb/encrypted_secrets: remove dependency to libnettle and use our own aes_gcm_128_*()

We already rely on gnutls in order to implement SSL/TLS, so using that
to speed up crypto like aes gcm 128 is fine, but as we already have
code for that algorithm, we should use that instead of adding a new
dependency to libnettle.

Some (I guess newer versions) of gnutls use nettle internally, so
we may end up using that code, but we should not have a direct dependency.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13276

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>