Removed Schwarz counter that should no longer be necessary.

The Schwarz counter in vtkTimeStamp is no longer needed. Now
the atomic integer classes don't manage any pointers unless
native atomic classes don't exist. In that case, I added a check
for NULL pointers which should handle the case where the global
counter is deleted at exit time.

Change-Id: I038b27fc7fcd43d4c7024108f6ff3808e97b2b3f
Merge topic 'remove-schwarz-counter' into master

47ca0348 Removed Schwarz counter that should no longer be necessary.