Replaced buggy and redundant implementation of GetRGBPoints()

This method was meant to return the XRGB points defining the color
transfer function, which is duplicate functionality with the method
GetDataPointer() in the superclass. Replaced the buggy implementation
(which returned the red color component in place of the transfer
function value) with an implementation that just calls
GetDataPointer().

In addition:

- Marked GetRGBPoints() as deprecated and suggested using
  GetDataPointer() instead.

- Removed a member variable no longer needed after this change.

- Added test specifically for vtkDiscretizableColorTransfer function
  that tests this change.

Change-Id: I98391c44712f12cd14dc57a0733bd66e595fa64b
Merge topic 'fix_discretizable_tf' into master

a4af85b3 Replaced buggy and redundant implementation of GetRGBPoints()