sql: remove unnecessary AUTOINCREMENT ID generation

Currently, if we perform something like
CREATE TABLE t1 (
        s1 INTEGER PRIMARY KEY AUTOINCREMENT,
        s2 INTEGER,
        CHECK (s1 <> 19)
);
INSERT INTO t1 VALUES (18, NULL);
INSERT INTO t1 (s2) VALUES (NULL);

we generate a new identifier in VDBE, but in any other case we
generate it in BOX. That was needed since the CHECK did not work
properly. This is not necessary now, because CHECK was moved to
BOX due to issue #3691. After this patch all new identifiers will
be generated in BOX.

Part of #4188

src/box/sql/insert.c

@@ -628,10 +628,7 @@ sqlInsert(Parse * pParse,	/* Parser context */
 			if (j < 0 || nColumn == 0
 			    || (pColumn && j >= pColumn->nId)) {
 				if (i == (int) autoinc_fieldno) {
-					sqlVdbeAddOp2(v,
-							  OP_NextAutoincValue,
-							  space->def->id,
-							  iRegStore);
+					sqlVdbeAddOp2(v, OP_Null, 0, iRegStore);
 					continue;
 				}
 				struct Expr *dflt = NULL;