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.

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>
(cherry picked from commit ee950e7ecf578bd225eec29b182f108450c5bfaf)

Change-Id: Idc2fa790d97b75971c29eec98bb96e799356dc4a
Reviewed-on: http://gerrit.openafs.org/5070
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>