Remove VC6 comment and pointless #ifdef'd benchmark code

We're in a wholly different world now, C++-compiler-wise.

Current std::stringstream implementations don't have the stated problem anymore,
and are just as fast as CDataStream.

The #ifdef'd block does not even compile anymore; CDataStream constructor changed,
and missing some std::. Also timing in whole seconds is also way too granular
to say anything sensible in such microbenchmarks. Just remove it,
it can always be found again in git history.