Removed assignment operator from vtkQuaternion

This wasn't needed, and is not implemented by any of the other classes
deriving from vtkTuple.

Change-Id: I00c36e7b6384cd14f6076e8cafcb16982dd136c3
Removed explicit destructors for vtkAtom and vtkBond

Neither was needed as they were both empty and did nothing - the
compiler generated destructor can do this and it fixes the Clang++
warnings on the dashboards.

Change-Id: If5bbaa77669058c82f8ad6a66256a9b712aa753e
Merge topic 'implicit-copy-constructor' into master

632a9242 Removed explicit destructors for vtkAtom and vtkBond
9007d128 Removed assignment operator from vtkQuaternion