Further simplify nbtree high key truncation.

 Further simplify nbtree high key truncation.
 
 Commit 7c2dbc69 reorganized _bt_truncate() in a way that enables a
 further simplification that I (pgeoghegan) missed:  Since we mark the
 tuple that is returned to the caller as a pivot tuple before the point
 where its heap TID is set as of 7c2dbc69, it is possible to use the high
 level BTreeTupleGetHeapTID() inline function to get an item pointer.  Do
 it that way now.  This approach is clearer and more maintainable.