Fix clang warning about comparision that's always true.

Specifically:
/Common/DataModel/vtkGenericAttributeCollection.cxx:456:44: warning: comparison of array 'this->AttributesToInterpolate' not equal to a null pointer is always true [-Wtautological-pointer-compare]
                                  || this->AttributesToInterpolate!=0));
                                     ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~  ~

The warning is correct, so remove the unneeded check.  Also, it's only in an assert.

Change-Id: I74fa877da2b167ff32f2a568c3e9f71db82581ff
Merge topic 'Wtautological-warning' into master

985aab49 Fix clang warning about comparision that's always true.