Don't log normal login_getpwclass(3) result.

The logging was introduced in r314527 but doesn't appear to be useful
for regular operation, and as the result, for users with no class set
(very common) the administrator would see a message like this in their
auth.log:

sshd[44251]: user root login class [preauth]

(note that the class was "" because that's what's typically configured
for most users; we would get 'default' if lc->lc_class is chosen)

Remove this log as it can be annoying as the lookup happen before
authentication and repeats, and our code is not acting upon lc_class
or pw_class directly anyways.

Reviewed by: cem, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24997