macos: krb5_524 is uselessly stubbed

macos: krb5_524 is uselessly stubbed

from MITKerberosShim-44:
dummy(krb5_524_conv_principal, 0);
dummy(krb5_524_convert_creds, 0);

this basically logs and returns success. not helpful.
so let's just not call them.

Change-Id: I91e7113e6934c67129e61dfc84b7f0c4d110cea0
Reviewed-on: http://gerrit.openafs.org/5066
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>