BUG: Vertext list leak

vtkPLY allocates a list of vertices for each face. That list was not
freed after it was used in vtkPLYReader.

To further compilate things, vtkPLY::ascii_get_element and
vtkPLY::binary_get_element used two different forms of heap allocation.

Change-Id: Ic92d3b9dd3291576a8ce285e905db760cbf7f23b
Merge topic 'PLYReader_Leak' into master

af106289 BUG: Vertext list leak