Remove unused MeasurementDataFlags mechanism

Given the choice between porting this placeholder to C++ or simply removing it,
I think it's better to remove it.  We can add it back in later if we have a need for it,
but for now it seems superfluous.
Don't pass current PluginMap to new plugins upon construction

None of them were using this currently, it's likely none ever will, and it would be
very difficult to support this for C++ algorithms.