fix bug in vtkRAdapter::VTKTreeToR(vtkTree* tree)

When this method was originally written it contained logic to
skip an extra root node in the vtkTree as it translated it into
an R variable.  More recently, I discovered that this extra root
node was being created by a bug in vtkNewickTreeReader.  Now that
the bug in the reader has been fixed, we need to eliminate this
"skip root node" logic from vtkRAdapter so that it will continue
to function as it has in the past.

Change-Id: Id269a91df7cb7e88f39a9e0bf92fa010a3a0b7af
Merge topic 'fix_RCalc' into master

5bd3962d fix bug in vtkRAdapter::VTKTreeToR(vtkTree* tree)