Avoid ROOT6 limitation by replaceing LinkDef files with xml specs.

In ROOT6, LinkDef.h files for specifying dictionaries do not work if the header defining the class is included in the LinkDef file, unless other changes are made.  Philippe Canal and I believe that the best course of action for CMS is to replace the LinkDef files with specifications in a classes_def.xml file.
This pull request replaces one particular LinkDef file that caused failures in many ROOT6 relval tests.
For reasons too long to include here, the error showed up with at least two different error messages,
one of which is:
Could not find the StreamerInfo for version 9 of the class TBranchElement, object skipped at offset 344

Because replacing the LinkDef file is completely compatible with ROOT 5, this fix is being requested in the main 7_1_X branch.

Please merge in a timely manner unless this request causes problems.