REMOVE NESTED CANCELLATION POINT SUPPORT

```
Description: The current implementation support nested cancellation points.
The TCB field cpcount keeps track of that nesting level.
However, cancellation points should not be calling other
cancellation points so this design could be simplified by
removing all support for nested cancellation points.
Status: Open
Priority: Low. No harm is being done by the current implementation.
This change is primarily for aesthetic reasons. If would
reduce memory usage by a very small but probably
insignificant amount.
```