Remove unused rbtnode->rpz flag

Merged Tony Finch requested to merge fanf-unused-rbtnode-rpz into main

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.

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