auth: remove stale "magic number" comment

auth: remove stale "magic number" comment

A comment in GenericAuth() refers to a "magic number" which used to be
present as:

  *aindex = 2;

Commit d5622d03196762bd8a60404fea98b4bb044e076d made this a proper enum:

  *aindex = RX_SECIDX_KAD;

Update the comment to remove mention of a "magic number".

No functional change is incurred by this commit.

Change-Id: I1d4770211fe4f88822426a9fe19db77bbb0d7738
Reviewed-on: https://gerrit.openafs.org/13490
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>