mdbx: remove pp_txnid4chk(), preparing to rework of page checking/validation.

src/internals.h

@@ -511,7 +511,8 @@ typedef struct MDBX_page {
 #define IS_SHADOWED(txn, p) ((p)->mp_txnid > (txn)->mt_txnid)
 #define IS_VALID(txn, p) ((p)->mp_txnid <= (txn)->mt_front)
 #define IS_MODIFIABLE(txn, p) ((p)->mp_txnid == (txn)->mt_front)
-    uint64_t mp_txnid;
+    uint64_t
+        mp_txnid; /* txnid which created this page, maybe zero in legacy DB */
     struct MDBX_page *mp_next; /* for in-memory list of freed pages */
   };
   uint16_t mp_leaf2_ksize; /* key size if this is a LEAF2 page */