Kerberos: remove debug logs when loading a keytab

Bug: 10543
Change-Id: I9ea20cd4d046cc072c21fd3f87f6d6be40b08b27
Reviewed-on: https://code.wireshark.org/review/4567
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>