Remove define for KRB5_CKSUM_LENGTH, which will become enctype-dependent

cleanup: When adding new encryption types, the checksum length
can be different for each enctype.  Face the fact that the
current code only supports DES which has a checksum length of 8.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>