Remove default_checksum and safe_checksum_type from krb5.conf

default_checksum was introduced back when DES is the only supported enctypes. Since then, more encryption types were added and each one has a single matched checksum type. There is no need to keep supporting this setting today.

safe_checksum_type is used to specify the default checksum type used in a KRB_SAFE message. In Java we do not use this message type at all. It should be removed as well.