tdb: remove seqnum field from header

Unused since " tdb: no longer need seqnum store hack as we can read after prepare."

ccan/tdb/tdb_private.h

@@ -231,7 +231,6 @@ struct tdb_context {
 	int transaction_lock_count;
 #ifdef TDB_TRACE
 	int tracefd;
-	uint32_t transaction_prepare_seqnum;
 #endif
 	volatile sig_atomic_t *interrupt_sig_ptr;
 };