make minor cleanups to print-decnet.c

make minor cleanups to print-decnet.c

Remove some unused code guarded by PRINT_NSPDATA, which was never
defined.

To resolve a Coverity dead code warning remove the notions of
COS_CRYPTSER as DECnet packet diagrams are pretty much non-existent in
the modern Internet so I could not figure it out what the code point was
meant to do originally.

dead_error_condition: The switch value "services & 0xc" cannot be 16