Remove unused rbtnode->rpz flag

Remove unused rbtnode->rpz flag

The rbtnode->rpz flag was left behind when rbt and rpz were disentangled
by CHANGES #4576. Removing it makes the comment above correct again.

This reduces the flags so they fit in a 32 bit word again. On 64
bit systems there is still padding so it doesn't change the size
of an rbtnode. On 32 bit systems it reduces an rbtnode by 4 bytes.