tdb2: remove tdb_null

We don't need this now, since we use explicit error numbers.

ccan/tdb2/tdb.c

@@ -2,9 +2,6 @@
 #include <ccan/asprintf/asprintf.h>
 #include <stdarg.h>
 
-/* The null return. */
-struct tdb_data tdb_null = { .dptr = NULL, .dsize = 0 };
-
 static enum TDB_ERROR update_rec_hdr(struct tdb_context *tdb,
 				     tdb_off_t off,
 				     tdb_len_t keylen,

ccan/tdb2/tdb2.h

@@ -600,11 +600,6 @@ union tdb_attribute {
 	struct tdb_attribute_stats stats;
 };
 
-/**
- * tdb_null - a convenient value for errors.
- */
-extern struct tdb_data tdb_null;
-
 #ifdef  __cplusplus
 }
 #endif