Remove workaround for fixed ROOT6 bug

There was a bug in ROOT6 that caused fatal errors if certain non-persistent classes did not have a default ctor,
We added default ctor's in the ROOT6 branch to two classes to avoid the problem.
The ROOT6 bug has now been fixed, and the fix is in the CMSSW ROOT6 IB.
This pull request removes the workaround, which is no longer needed.