pgsql: Remove special-handling of usernames with Kerberos

Log Message:
-----------
Remove special-handling of usernames with Kerberos authentication. We will
now always use the system username as the default, and not try to pick it up
from the kerberos ticket.

This fixes the spurious error messages that show up on kerberos-enabled builds
when not actually using kerberos, and puts it in line with how other authentication
methods work.

Modified Files:
--------------
pgsql/src/interfaces/libpq:
fe-auth.c (r1.139 -> r1.140)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c?r1=1.139&r2=1.140)