Fix memory leak when getting a tree from RCalculatorFilter's output

This commit updates vtkRAdapter::RToVTKTree() to match the memory
management strategy used in vtkRAdapter::RToVTKTable().  This commit
also removes the deletion of the vtkTree in TestRInterface, as this
is rendered inappropriate by the previously mentioned change.

Change-Id: I764c9d28199d7b5421383947567cd37a93dbdec3
Merge topic 'fix_R_tree_memory_leak' into master

3fee36a7 Fix memory leak when getting a tree from RCalculatorFilter's output