remote: Drop KRB5_KTNAME override

remote: Drop KRB5_KTNAME override

When the comment in libvirtd.sasl was last updated with

  commit fe772f24a6809b3d937ed6547cbaa9d820e514b6
  Author: Cole Robinson <crobinso@redhat.com>
  Date:   Sat Oct 20 14:10:03 2012 -0400

    daemon: Avoid 'Could not find keytab file' in syslog

it was noted that only old versions of kerberos would need the
environment variable to be set: that was more than seven years
ago, so it's safe to assume that none of our current target
platforms still requires that hack and setting the appropriate
key in the configuration file will be enough.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>