rxrpc: Don't bother generating maxSkew in the ACK packet

commit e8c3af6b upstream.

Don't bother generating maxSkew in the ACK packet as it has been obsolete
since AFS 3.1.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>